Choose any three.
|
|
PRAGMA Statements
The PRAGMA statement is an SQL extension specific to SQLite and used to modify the operation of the SQLite library or to query the SQLite library for internal (non-table) data. The PRAGMA statement is issued using the same interface as other SQLite commands (e.g. SELECT, INSERT) but is different in the following important respects:
- Specific pragma statements may be removed and others added in future releases of SQLite. There is no guarantee of backwards compatibility.
- No error messages are generated if an unknown pragma is issued. Unknown pragmas are simply ignored. This means if there is a typo in a pragma statement the library does not inform the user of the fact.
- Some pragmas take effect during the SQL compilation stage, not the execution stage. This means if using the C-language sqlite3_prepare(), sqlite3_step(), sqlite3_finalize() API (or similar in a wrapper interface), the pragma may run during the sqlite3_prepare() call, not during the sqlite3_step() call as normal SQL statements do. Or the pragma might run during sqlite3_step() just like normal SQL statements. Whether or not the pragma runs during sqlite3_prepare() or sqlite3_step() depends on the pragma and on the specific release of SQLite.
- The pragma command is specific to SQLite and is very unlikely to be compatible with any other SQL database engine.
The C-language API for SQLite provides the SQLITE_FCNTL_PRAGMA file control which gives VFS implementations the opportunity to add new PRAGMA statements or to override the meaning of built-in PRAGMA statements.
PRAGMA command syntax
A pragma can take either zero or one argument. The argument is may be either in parentheses or it may be separated from the pragma name by an equal sign. The two syntaxes yield identical results. In many pragmas, the argument is a boolean. The boolean can be one of:
0 no false off
Keyword arguments can optionally appear in quotes. (Example: 'yes' [FALSE].) Some pragmas takes a string literal as their argument. When pragma takes a keyword argument, it will usually also take a numeric equivalent as well. For example, "0" and "no" mean the same thing, as does "1" and "yes". When querying the value of a setting, many pragmas return the number rather than the keyword.
A pragma may have an optional database name before the pragma name. The database name is the name of an ATTACH-ed database or it can be "main" or "temp" for the main and the TEMP databases. If the optional database name is omitted, "main" is assumed. In some pragmas, the database name is meaningless and is simply ignored.
List Of PRAGMAs
Notes:
- Pragmas whose names are marked through in the list above are deprecated that are maintained for historical compatibility only. Do not use the deprecated pragmas in new applications. Remove deprecated pragmas from existing applications at your earliest opportunity.
- These pragmas are used for debugging SQLite and are only available when SQLite is compiled using SQLITE_DEBUG.
- These pragmas are used for testing SQLite and are not recommended for use in application programs.
PRAGMA application_id;
PRAGMA application_id = integer ;
The application_id PRAGMA is used to query or set the 32-bit unsigned big-endian "Application ID" integer located at offset 68 into the database header. Applications that use SQLite as their application file-format should set the Application ID integer to a unique integer so that utilities such as file(1) can determine the specific file type rather than just reporting "SQLite3 Database". A list of assigned application IDs can be seen by consulting the magic.txt file in the SQLite source repository.
PRAGMA auto_vacuum;
PRAGMA auto_vacuum =
0 | NONE | 1 | FULL | 2 | INCREMENTAL;
Query or set the auto-vacuum status in the database.
The default setting for auto-vacuum is 0 or "none", unless the SQLITE_DEFAULT_AUTOVACUUM compile-time option is used. The "none" setting means that auto-vacuum is disabled. When auto-vacuum is disabled and data is deleted data from a database, the database file remains the same size. Unused database file pages are added to a "freelist" and reused for subsequent inserts. So no database file space is lost. However, the database file does not shrink. In this mode the VACUUM command can be used to rebuild the entire database file and thus reclaim unused disk space.
When the auto-vacuum mode is 1 or "full", the freelist pages are moved to the end of the database file and the database file is truncated to remove the freelist pages at every transaction commit. Note, however, that auto-vacuum only truncates the freelist pages from the file. Auto-vacuum does not defragment the database nor repack individual database pages the way that the VACUUM command does. In fact, because it moves pages around within the file, auto-vacuum can actually make fragmentation worse.
Auto-vacuuming is only possible if the database stores some additional information that allows each database page to be traced backwards to its referrer. Therefore, auto-vacuuming must be turned on before any tables are created. It is not possible to enable or disable auto-vacuum after a table has been created.
When the value of auto-vacuum is 2 or "incremental" then the additional information needed to do auto-vacuuming is stored in the database file but auto-vacuuming does not occur automatically at each commit as it does with auto_vacuum=full. In incremental mode, the separate incremental_vacuum pragma must be invoked to cause the auto-vacuum to occur.
The database connection can be changed between full and incremental autovacuum mode at any time. However, changing from "none" to "full" or "incremental" can only occur when the database is new (no tables have yet been created) or by running the VACUUM command. To change auto-vacuum modes, first use the auto_vacuum pragma to set the new desired mode, then invoke the VACUUM command to reorganize the entire database file. To change from "full" or "incremental" back to "none" always requires running VACUUM even on an empty database.
When the auto_vacuum pragma is invoked with no arguments, it returns the current auto_vacuum mode.
PRAGMA automatic_index;
PRAGMA automatic_index = boolean;
Query, set, or clear the automatic indexing capability.
Automatic indexing is enabled by default as of version 3.7.17, but this might change in future releases of SQLite.
PRAGMA busy_timeout;
PRAGMA busy_timeout = milliseconds;
Query or change the setting of the busy timeout. This pragma is an alternative to the sqlite3_busy_timeout() C-language interface which is made available as a pragma for use with language bindings that do not provide direct access to sqlite3_busy_timeout().
Each database connection can only have a single busy handler. This PRAGMA sets the busy handler for the process, possibly overwriting any previously set busy handler.
PRAGMA cache_size;
PRAGMA cache_size = pages;
PRAGMA cache_size = -kibibytes;
Query or change the suggested maximum number of database disk pages that SQLite will hold in memory at once per open database file. Whether or not this suggestion is honored is at the discretion of the Application Defined Page Cache. The default page cache that is built into SQLite honors the request, however alternative application-defined page cache implementations may choose to interpret the suggested cache size in different ways or to ignore it all together. The default suggested cache size is 2000 pages.
If the argument N is positive then the suggested cache size is set to N. If the argument N is negative, then the number of cache pages is adjusted to use approximately N*1024 bytes of memory. Backwards compatibility note: The behavior of cache_size with a negative N was different in SQLite versions prior to 3.7.10. In version 3.7.9 and earlier, the number of pages in the cache was set to the absolute value of N.
When you change the cache size using the cache_size pragma, the change only endures for the current session. The cache size reverts to the default value when the database is closed and reopened.
PRAGMA cache_spill;
PRAGMA cache_spill=boolean;
The cache_spill pragma enables or disables the ability of the pager to spill dirty cache pages to the database file in the middle of a transaction. Cache_spill is enabled by default and most applications should leave it that way as cache spilling is usually advantageous. However, a cache spill has the side-effect of acquiring an EXCLUSIVE lock on the database file. Hence, some applications that have large long-running transactions may want to disable cache spilling in order to prevent the application from acquiring an exclusive lock on the database until the moment that the transaction COMMITs.
PRAGMA case_sensitive_like = boolean;
The default behavior of the LIKE operator is to ignore case for ASCII characters. Hence, by default 'a' LIKE 'A' is true. The case_sensitive_like pragma installs a new application-defined LIKE function that is either case sensitive or insensitive depending on the value of the case_sensitive_like pragma. When case_sensitive_like is disabled, the default LIKE behavior is expressed. When case_sensitive_like is enabled, case becomes significant. So, for example, 'a' LIKE 'A' is false but 'a' LIKE 'a' is still true.
This pragma uses sqlite3_create_function() to overload the LIKE and GLOB functions, which may override previous implementations of LIKE and GLOB registered by the application.
PRAGMA checkpoint_fullfsync
PRAGMA checkpoint_fullfsync = boolean;
Query or change the fullfsync flag for checkpoint operations. If this flag is set, then the F_FULLFSYNC syncing method is used during checkpoint operations on systems that support F_FULLFSYNC. The default value of the checkpoint_fullfsync flag is off. Only Mac OS-X supports F_FULLFSYNC.
If the fullfsync flag is set, then the F_FULLFSYNC syncing method is used for all sync operations and the checkpoint_fullfsync setting is irrelevant.
PRAGMA collation_list;
Return a list of the collating sequences defined for the current database connection.
PRAGMA compile_options;
This pragma returns the names of compile-time options used when building SQLite, one option per row. The "SQLITE_" prefix is omitted from the returned option names. See also the sqlite3_compileoption_get() C/C++ interface and the sqlite_compileoption_get() SQL functions.
PRAGMA count_changes;
PRAGMA count_changes = boolean;
Query or change the count-changes flag. Normally, when the count-changes flag is not set, INSERT, UPDATE and DELETE statements return no data. When count-changes is set, each of these commands returns a single row of data consisting of one integer value - the number of rows inserted, modified or deleted by the command. The returned change count does not include any insertions, modifications or deletions performed by triggers, or any changes made automatically by foreign key actions.
Another way to get the row change counts is to use the sqlite3_changes() or sqlite3_total_changes() interfaces. There is a subtle different, though. When an INSERT, UPDATE, or DELETE is run against a view using an INSTEAD OF trigger, the count_changes pragma reports the number of rows in the view that fired the trigger, whereas sqlite3_changes() and sqlite3_total_changes() do not.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA data_store_directory;
PRAGMA data_store_directory = 'directory-name';
Query or change the value of the sqlite3_data_directory global variable, which windows operating-system interface backends use to determine where to store database files specified using a relative pathname.
Changing the data_store_directory setting is not threadsafe. Never change the data_store_directory setting if another thread within the application is running any SQLite interface at the same time. Doing so results in undefined behavior. Changing the data_store_directory setting writes to the sqlite3_data_directory global variable and that global variable is not protected by a mutex.
This facility is provided for WinRT which does not have an OS mechanism for reading or changing the current working directory. The use of this pragma in any other context is discouraged and may be disallowed in future releases.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA database_list;
This pragma works like a query to return one row for each database attached to the current database connection. The second column is the "main" for the main database file, "temp" for the database file used to store TEMP objects, or the name of the ATTACHed database for other database files. The third column is the name of the database file itself, or an empty string if the database is not associated with a file.
PRAGMA default_cache_size;
PRAGMA default_cache_size = Number-of-pages;
This pragma queries or sets the suggested maximum number of pages of disk cache that will be allocated per open database file. The difference between this pragma and cache_size is that the value set here persists across database connections. The value of the default cache size is stored in the 4-byte big-endian integer located at offset 48 in the header of the database file.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA defer_foreign_keys
PRAGMA defer_foreign_keys = boolean;
When the defer_foreign_keys PRAGMA is on, enforcement of all foreign key constraints is delayed until the outermost transaction is committed. The defer_foreign_keys pragma defaults to OFF so that foreign key constraints are only deferred if they are created as "DEFERRABLE INITIALLY DEFERRED". The defer_foreign_keys pragma is automatically switched off at each COMMIT or ROLLBACK. Hence, the defer_foreign_keys pragma must be separately enabled for each transaction. This pragma is only meaningful if foreign key constraints are enabled, of course.
The sqlite3_db_status(db,SQLITE_DBSTATUS_DEFERRED_FKS,...) C-language interface can be used during a transaction to determine if there are deferred and unresolved foreign key constraints.
PRAGMA empty_result_callbacks;
PRAGMA empty_result_callbacks = boolean;
Query or change the empty-result-callbacks flag.
The empty-result-callbacks flag affects the sqlite3_exec() API only. Normally, when the empty-result-callbacks flag is cleared, the callback function supplied to the sqlite3_exec() is not invoked for commands that return zero rows of data. When empty-result-callbacks is set in this situation, the callback function is invoked exactly once, with the third parameter set to 0 (NULL). This is to enable programs that use the sqlite3_exec() API to retrieve column-names even when a query returns no data.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA encoding;
PRAGMA encoding = "UTF-8";
PRAGMA encoding = "UTF-16";
PRAGMA encoding = "UTF-16le";
PRAGMA encoding = "UTF-16be";
In first form, if the main database has already been created, then this pragma returns the text encoding used by the main database, one of "UTF-8", "UTF-16le" (little-endian UTF-16 encoding) or "UTF-16be" (big-endian UTF-16 encoding). If the main database has not already been created, then the value returned is the text encoding that will be used to create the main database, if it is created by this session.
The second through fifth forms of this pragma set the encoding that the main database will be created with if it is created by this session. The string "UTF-16" is interpreted as "UTF-16 encoding using native machine byte-ordering". It is not possible to change the text encoding of a database after it has been created and any attempt to do so will be silently ignored.
Once an encoding has been set for a database, it cannot be changed.
Databases created by the ATTACH command always use the same encoding as the main database. An attempt to ATTACH a database with a different text encoding from the "main" database will fail.
PRAGMA foreign_key_check;
PRAGMA foreign_key_check(table-name);
The foreign_key_check pragma checks the database, or the table called "table-name", for foreign key constraints that are violated and returns one row of output for each violation. There are four columns in each result row. The first column is the name of the table that contains the REFERENCES clause. The second column is the rowid of the row that contains the invalid REFERENCES clause. The third column is the name of the table that is referred to. The fourth column is the index of the specific foreign key constraint that failed. The fourth column in the output of the foreign_key_check pragma is the same integer as the first column in the output of the foreign_key_list pragma. When a "table-name" is specified, the only foreign key constraints checked are those created by REFERENCES clauses in the CREATE TABLE statement for table-name.
PRAGMA foreign_key_list(table-name);
This pragma returns one row for each foreign key constraint created by a REFERENCES clause in the CREATE TABLE statement of table "table-name".
PRAGMA foreign_keys;
PRAGMA foreign_keys = boolean;
Query, set, or clear the enforcement of foreign key constraints.
This pragma is a no-op within a transaction; foreign key constraint enforcement may only be enabled or disabled when there is no pending BEGIN or SAVEPOINT.
Changing the foreign_keys setting affects the execution of all statements prepared using the database connection, including those prepared before the setting was changed. Any existing statements prepared using the legacy sqlite3_prepare() interface may fail with an SQLITE_SCHEMA error after the foreign_keys setting is changed.
As of SQLite version 3.6.19, the default setting for foreign key enforcement is OFF. However, that might change in a future release of SQLite. The default setting for foreign key enforcement can be specified at compile-time using the SQLITE_DEFAULT_FOREIGN_KEYS preprocessor macro. To minimize future problems, applications should set the foreign key enforcement flag as required by the application and not depend on the default setting.
PRAGMA freelist_count;
Return the number of unused pages in the database file.
PRAGMA full_column_names;
PRAGMA full_column_names = boolean;
Query or change the full_column_names flag. This flag together with the short_column_names flag determine the way SQLite assigns names to result columns of SELECT statements. Result columns are named by applying the following rules in order:
If there is an AS clause on the result, then the name of the column is the right-hand side of the AS clause.
If the result is a general expression, not a just the name of a source table column, then the name of the result is a copy of the expression text.
If the short_column_names pragma is ON, then the name of the result is the name of the source table column without the source table name prefix: COLUMN.
If both pragmas short_column_names and full_column_names are OFF then case (2) applies.
The name of the result column is a combination of the source table and source column name: TABLE.COLUMN
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA fullfsync
PRAGMA fullfsync = boolean;
Query or change the fullfsync flag. This flag determines whether or not the F_FULLFSYNC syncing method is used on systems that support it. The default value of the fullfsync flag is off. Only Mac OS X supports F_FULLFSYNC.
See also checkpoint_fullfsync.
PRAGMA ignore_check_constraints = boolean;
This pragma enables or disables the enforcement of CHECK constraints. The default setting is off, meaning that CHECK constraints are enforced by default.
PRAGMA incremental_vacuum(N);
The incremental_vacuum pragma causes up to N pages to be removed from the freelist. The database file is truncated by the same amount. The incremental_vacuum pragma has no effect if the database is not in auto_vacuum=incremental mode or if there are no pages on the freelist. If there are fewer than N pages on the freelist, or if N is less than 1, or if N is omitted entirely, then the entire freelist is cleared.
PRAGMA index_info(index-name);
This pragma returns one row each column in the named index. The first column of the result is the rank of the column within the index. The second column of the result is the rank of the column within the table. The third column of output is the name of the column being indexed.
PRAGMA index_list(table-name);
This pragma returns one row for each index associated with the given table. Columns of the result set include the index name and a flag to indicate whether or not the index is UNIQUE.
PRAGMA integrity_check;
PRAGMA integrity_check(N)
This pragma does an integrity check of the entire database. The integrity_check pragma looks for out-of-order records, missing pages, malformed records, missing index entries, and UNIQUE and NOT NULL constraint errors. If the integrity_check pragma finds problems, strings are returned (as multiple rows with a single column per row) which describe the problems. Pragma integrity_check will return at most N errors before the analysis quits, with N defaulting to 100. If pragma integrity_check finds no errors, a single row with the value 'ok' is returned.
PRAGMA integrity_check does not find FOREIGN KEY errors. Use the PRAGMA foreign_key_check command for to find errors in FOREIGN KEY constraints.
See also the PRAGMA quick_check command which does most of the checking of PRAGMA integrity_check but runs much faster.
PRAGMA journal_mode;
PRAGMA database.journal_mode;
PRAGMA journal_mode
= DELETE | TRUNCATE | PERSIST | MEMORY | WAL | OFF
PRAGMA database.journal_mode
= DELETE | TRUNCATE | PERSIST | MEMORY | WAL | OFF
This pragma queries or sets the journal mode for databases associated with the current database connection.
The first two forms of this pragma query the current journaling mode for database. When database is omitted, the "main" database is queried.
The last two forms change the journaling mode. The 4th form changes the journaling mode for a specific database connection named. Use "main" for the main database (the database that was opened by the original sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2() interface call) and use "temp" for database that holds TEMP tables. The 3rd form changes the journaling mode on all databases attached to the connection. The new journal mode is returned. If the journal mode could not be changed, the original journal mode is returned.
The DELETE journaling mode is the normal behavior. In the DELETE mode, the rollback journal is deleted at the conclusion of each transaction. Indeed, the delete operation is the action that causes the transaction to commit. (See the document titled Atomic Commit In SQLite for additional detail.)
The TRUNCATE journaling mode commits transactions by truncating the rollback journal to zero-length instead of deleting it. On many systems, truncating a file is much faster than deleting the file since the containing directory does not need to be changed.
The PERSIST journaling mode prevents the rollback journal from being deleted at the end of each transaction. Instead, the header of the journal is overwritten with zeros. This will prevent other database connections from rolling the journal back. The PERSIST journaling mode is useful as an optimization on platforms where deleting or truncating a file is much more expensive than overwriting the first block of a file with zeros. See also: PRAGMA journal_size_limit and SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT.
The MEMORY journaling mode stores the rollback journal in volatile RAM. This saves disk I/O but at the expense of database safety and integrity. If the application using SQLite crashes in the middle of a transaction when the MEMORY journaling mode is set, then the database file will very likely go corrupt.
The WAL journaling mode uses a write-ahead log instead of a rollback journal to implement transactions. The WAL journaling mode is persistent; after being set it stays in effect across multiple database connections and after closing and reopening the database. A database in WAL journaling mode can only be accessed by SQLite version 3.7.0 or later.
The OFF journaling mode disables the rollback journal completely. No rollback journal is ever created and hence there is never a rollback journal to delete. The OFF journaling mode disables the atomic commit and rollback capabilities of SQLite. The ROLLBACK command no longer works; it behaves in an undefined way. Applications must avoid using the ROLLBACK command when the journal mode is OFF. If the application crashes in the middle of a transaction when the OFF journaling mode is set, then the database file will very likely go corrupt.
Note that the journal_mode for an in-memory database is either MEMORY or OFF and can not be changed to a different value. An attempt to change the journal_mode of an in-memory database to any setting other than MEMORY or OFF is ignored. Note also that the journal_mode cannot be changed while a transaction is active.
PRAGMA journal_size_limit
PRAGMA journal_size_limit = N ;
If a database connection is operating in exclusive locking mode or in persistent journal mode (PRAGMA journal_mode=persist) then after committing a transaction the rollback journal file may remain in the file-system. This increases performance for subsequent transactions since overwriting an existing file is faster than append to a file, but it also consumes file-system space. After a large transaction (e.g. a VACUUM), the rollback journal file may consume a very large amount of space.
Similarly, in WAL mode, the write-ahead log file is not truncated following a checkpoint. Instead, SQLite reuses the existing file for subsequent WAL entries since overwriting is faster than appending.
The journal_size_limit pragma may be used to limit the size of rollback-journal and WAL files left in the file-system after transactions or checkpoints. Each time a transaction is committed or a WAL file resets, SQLite compares the size of the rollback journal file or WAL file left in the file-system to the size limit set by this pragma and if the journal or WAL file is larger it is truncated to the limit.
The second form of the pragma listed above is used to set a new limit in bytes for the specified database. A negative number implies no limit. To always truncate rollback journals and WAL files to their minimum size, set the journal_size_limit to zero. Both the first and second forms of the pragma listed above return a single result row containing a single integer column - the value of the journal size limit in bytes. The default journal size limit is -1 (no limit). The SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT preprocessor macro can be used to change the default journal size limit at compile-time.
This pragma only operates on the single database specified prior to the pragma name (or on the "main" database if no database is specified.) There is no way to change the journal size limit on all attached databases using a single PRAGMA statement. The size limit must be set separately for each attached database.
PRAGMA legacy_file_format;
PRAGMA legacy_file_format = boolean
This pragma sets or queries the value of the legacy_file_format flag. When this flag is on, new SQLite databases are created in a file format that is readable and writable by all versions of SQLite going back to 3.0.0. When the flag is off, new databases are created using the latest file format which might not be readable or writable by versions of SQLite prior to 3.3.0.
When the legacy_file_format pragma is issued with no argument, it returns the setting of the flag. This pragma does not tell which file format the current database is using; it tells what format will be used by any newly created databases.
The legacy_file_format pragma is initialized to OFF when an existing database in the newer file format is first opened.
The default file format is set by the SQLITE_DEFAULT_FILE_FORMAT compile-time option.
PRAGMA locking_mode;
PRAGMA locking_mode = NORMAL | EXCLUSIVE
This pragma sets or queries the database connection locking-mode. The locking-mode is either NORMAL or EXCLUSIVE.
In NORMAL locking-mode (the default unless overridden at compile-time using SQLITE_DEFAULT_LOCKING_MODE), a database connection unlocks the database file at the conclusion of each read or write transaction. When the locking-mode is set to EXCLUSIVE, the database connection never releases file-locks. The first time the database is read in EXCLUSIVE mode, a shared lock is obtained and held. The first time the database is written, an exclusive lock is obtained and held.
Database locks obtained by a connection in EXCLUSIVE mode may be released either by closing the database connection, or by setting the locking-mode back to NORMAL using this pragma and then accessing the database file (for read or write). Simply setting the locking-mode to NORMAL is not enough - locks are not released until the next time the database file is accessed.
There are three reasons to set the locking-mode to EXCLUSIVE.
- The application wants to prevent other processes from accessing the database file.
- The number of system calls for filesystem operations is reduced, possibly resulting in a small performance increase.
- WAL databases can be accessed in EXCLUSIVE mode without the use of shared memory. (Additional information)
When the locking_mode pragma specifies a particular database, for example:
PRAGMA main.locking_mode=EXCLUSIVE;
Then the locking mode applies only to the named database. If no database name qualifier precedes the "locking_mode" keyword then the locking mode is applied to all databases, including any new databases added by subsequent ATTACH commands.
The "temp" database (in which TEMP tables and indices are stored) and in-memory databases always uses exclusive locking mode. The locking mode of temp and in-memory databases cannot be changed. All other databases use the normal locking mode by default and are affected by this pragma.
If the locking mode is EXCLUSIVE when first entering WAL journal mode, then the locking mode cannot be changed to NORMAL until after exiting WAL journal mode. If the locking mode is NORMAL when first entering WAL journal mode, then the locking mode can be changed between NORMAL and EXCLUSIVE and back again at any time and without needing to exit WAL journal mode.
PRAGMA max_page_count;
PRAGMA max_page_count = N;
Query or set the maximum number of pages in the database file. Both forms of the pragma return the maximum page count. The second form attempts to modify the maximum page count. The maximum page count cannot be reduced below the current database size.
PRAGMA database.mmap_size;
PRAGMA database.mmap_size=N
Query or change the maximum number of bytes that are set aside for memory-mapped I/O on a single database. The first form (without an argument) queries the current limit. The second form (with a numeric argument) sets the limit for the specified database, or for all databases if the optional database name is omitted. In the second form, if the database name is omitted, the limit that is set becomes the default limit for all databases that are added to the database connection by subsequent ATTACH statements.
The argument N is the maximum number of bytes of the database file that will be accessed using memory-mapped I/O. If N is zero then memory mapped I/O is disabled. If N is negative, then the limit reverts to the default value determined by the most recent sqlite3_config(SQLITE_CONFIG_MMAP_SIZE), or to the compile time default determined by SQLITE_DEFAULT_MMAP_SIZE if not start-time limit has been set.
The PRAGMA mmap_size statement will never increase the amount of address space used for memory-mapped I/O above the hard limit set by the SQLITE_MAX_MMAP_SIZE compile-time option, nor the hard limit set start-time by the second argument to sqlite3_config(SQLITE_CONFIG_MMAP_SIZE)
The size of the memory-mapped I/O region cannot be changed while the memory-mapped I/O region is in active use, to avoid unmapping memory out from under running SQL statements. For this reason, the mmap_size pragma may be a no-op if the prior mmap_size is non-zero and there are other SQL statements running concurrently on the same database connection.
PRAGMA page_count;
Return the total number of pages in the database file.
PRAGMA page_size;
PRAGMA page_size = bytes;
Query or set the page size of the database. The page size must be a power of two between 512 and 65536 inclusive.
When a new database is created, SQLite assigned a default page size based on information received from the xSectorSize and xDeviceCharacteristics methods of the sqlite3_io_methods object of the newly created database file. The page_size pragma will only cause an immediate change in the page size if it is issued while the database is still empty, prior to the first CREATE TABLE statement. If the page_size pragma is used to specify a new page size just prior to running the VACUUM command and if the database is not in WAL journal mode then VACUUM will change the page size to the new value.
If SQLite is compiled with the SQLITE_ENABLE_ATOMIC_WRITE option, then the default page size is chosen to be the largest page size less than or equal to SQLITE_MAX_DEFAULT_PAGE_SIZE for which atomic write is enabled according to the xDeviceCharacteristics method of the sqlite3_io_methods object for the database file. If the SQLITE_ENABLE_ATOMIC_WRITE option is disabled or if xDeviceCharacteristics reports no suitable atomic write page sizes, then the default page size is the larger of SQLITE_DEFAULT_PAGE_SIZE and the sector size as reported by the xSectorSize method of the sqlite3_io_methods object, but not more than SQLITE_MAX_DEFAULT_PAGE_SIZE. The normal configuration for SQLite running on workstations is for atomic write to be disabled, for the maximum page size to be set to 65536, for SQLITE_DEFAULT_PAGE_SIZE to be 1024, and for the maximum default page size to be set to 8192. The default xSectorSize method on unix workstation implementations always reports a sector size of 512 bytes. Hence, the default page size chosen by SQLite on unix is usually 1024 bytes. On windows, the GetDiskFreeSpace() interface is used to obtain the actual device sector size and hence the default page size on windows will sometimes be greater than 1024.
PRAGMA parser_trace = boolean;
If SQLite has been compiled with the SQLITE_DEBUG compile-time option, then the parser_trace pragma can be used to turn on tracing for the SQL parser used internally by SQLite. This feature is used for debugging SQLite itself.
This pragma is intended for use when debugging SQLite itself. It is only available when the SQLITE_DEBUG compile-time option is used.
PRAGMA query_only;
PRAGMA query_only = boolean;
The query_only pragma prevents all changes to database files when enabled.
PRAGMA quick_check;
PRAGMA quick_check(N)
The pragma is like integrity_check except that it does not verify UNIQUE and NOT NULL constraints and does not verify that index content matches table content. By skipping UNIQUE and NOT NULL and index consistency checks, quick_check is able to run much faster than integrity_check. Otherwise the two pragmas are the same.
PRAGMA read_uncommitted;
PRAGMA read_uncommitted = boolean;
Query, set, or clear READ UNCOMMITTED isolation. The default isolation level for SQLite is SERIALIZABLE. Any process or thread can select READ UNCOMMITTED isolation, but SERIALIZABLE will still be used except between connections that share a common page and schema cache. Cache sharing is enabled using the sqlite3_enable_shared_cache() API. Cache sharing is disabled by default.
See SQLite Shared-Cache Mode for additional information.
PRAGMA recursive_triggers;
PRAGMA recursive_triggers = boolean;
Query, set, or clear the recursive trigger capability.
Changing the recursive_triggers setting affects the execution of all statements prepared using the database connection, including those prepared before the setting was changed. Any existing statements prepared using the legacy sqlite3_prepare() interface may fail with an SQLITE_SCHEMA error after the recursive_triggers setting is changed.
Prior to SQLite version 3.6.18, recursive triggers were not supported. The behavior of SQLite was always as if this pragma was set to OFF. Support for recursive triggers was added in version 3.6.18 but was initially turned OFF by default, for compatibility. Recursive triggers may be turned on by default in future versions of SQLite.
The depth of recursion for triggers has a hard upper limit set by the SQLITE_MAX_TRIGGER_DEPTH compile-time option and a run-time limit set by sqlite3_limit(db,SQLITE_LIMIT_TRIGGER_DEPTH,...).
PRAGMA reverse_unordered_selects;
PRAGMA reverse_unordered_selects = boolean;
When enabled, this PRAGMA causes SELECT statements without an ORDER BY clause to emit their results in the reverse order of what they normally would. This can help debug applications that are making invalid assumptions about the result order.
SQLite makes no guarantees about the order of results if a SELECT omits the ORDER BY clause. Even so, the order of results does not change from one run to the next, and so many applications mistakenly come to depend on the arbitrary output order whatever that order happens to be. However, sometimes new versions of SQLite will contain optimizer enhancements that will cause the output order of queries without ORDER BY clauses to shift. When that happens, applications that depend on a certain output order might malfunction. By running the application multiple times with this pragma both disabled and enabled, cases where the application makes faulty assumptions about output order can be identified and fixed early, reducing problems that might be caused by linking against a different version of SQLite.
PRAGMA schema_version;
PRAGMA schema_version = integer ;
PRAGMA user_version;
PRAGMA user_version = integer ;
The pragmas schema_version and user_version are used to set or get the value of the schema-version and user-version, respectively. The schema-version and the user-version are big-endian 32-bit signed integers stored in the database header at offsets 40 and 60, respectively.
The schema-version is usually only manipulated internally by SQLite. It is incremented by SQLite whenever the database schema is modified (by creating or dropping a table or index). The schema version is used by SQLite each time a query is executed to ensure that the internal cache of the schema used when compiling the SQL query matches the schema of the database against which the compiled query is actually executed. Subverting this mechanism by using "PRAGMA schema_version" to modify the schema-version is potentially dangerous and may lead to program crashes or database corruption. Use with caution!
The user-version is not used internally by SQLite. It may be used by applications for any purpose.
PRAGMA secure_delete;
PRAGMA database.secure_delete;
PRAGMA secure_delete = boolean
PRAGMA database.secure_delete =
boolean
Query or change the secure-delete setting. When secure-delete on, SQLite overwrites deleted content with zeros. The default setting is determined by the SQLITE_SECURE_DELETE compile-time option.
When there are attached databases and no database is specified in the pragma, all databases have their secure-delete setting altered. The secure-delete setting for newly attached databases is the setting of the main database at the time the ATTACH command is evaluated.
When multiple database connections share the same cache, changing the secure-delete flag on one database connection changes it for them all.
PRAGMA short_column_names;
PRAGMA short_column_names = boolean;
Query or change the short-column-names flag. This flag affects the way SQLite names columns of data returned by SELECT statements. See the full_column_names pragma for full details.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA shrink_memory
This pragma causes the database connection on which it is invoked to free up as much memory as it can, by calling sqlite3_db_release_memory().
PRAGMA soft_heap_limit
PRAGMA soft_heap_limit=N
This pragma invokes the sqlite3_soft_heap_limit64() interface with the argument N, if N is specified and is a non-negative integer. The soft_heap_limit pragma always returns the same integer that would be returned by the sqlite3_soft_heap_limit64(-1) C-language function.
PRAGMA stats;
This pragma returns auxiliary information about tables and indices. The returned information is used during testing to help verify that the query planner is operating correctly. The format and meaning of this pragma will likely change from release to the next. Because of its volatility, the behavior and output format of this pragma are deliberately undocumented.
The intended use of this pragma is only for testing and validation of SQLite. This pragma is subject to change without notice and is not recommended for use by application programs.
PRAGMA synchronous;
PRAGMA synchronous =
0 | OFF | 1 | NORMAL | 2 | FULL;
Query or change the setting of the "synchronous" flag. The first (query) form will return the synchronous setting as an integer. When synchronous is FULL (2), the SQLite database engine will use the xSync method of the VFS to ensure that all content is safely written to the disk surface prior to continuing. This ensures that an operating system crash or power failure will not corrupt the database. FULL synchronous is very safe, but it is also slower. When synchronous is NORMAL (1), the SQLite database engine will still sync at the most critical moments, but less often than in FULL mode. There is a very small (though non-zero) chance that a power failure at just the wrong time could corrupt the database in NORMAL mode. But in practice, you are more likely to suffer a catastrophic disk failure or some other unrecoverable hardware fault. With synchronous OFF (0), SQLite continues without syncing as soon as it has handed data off to the operating system. If the application running SQLite crashes, the data will be safe, but the database might become corrupted if the operating system crashes or the computer loses power before that data has been written to the disk surface. On the other hand, some operations are as much as 50 or more times faster with synchronous OFF.
In WAL mode when synchronous is NORMAL (1), the WAL file is synchronized before each checkpoint and the database file is synchronized after each completed checkpoint and the WAL file header is synchronized when a WAL file begins to be reused after a checkpoint, but no sync operations occur during most transactions. With synchronous=FULL in WAL mode, an additional sync operation of the WAL file happens after each transaction commit. The extra WAL sync following each transaction help ensure that transactions are durable across a power loss, but they do not aid in preserving consistency. If durability is not a concern, then synchronous=NORMAL is normally all one needs in WAL mode.
The default setting is synchronous=FULL.
See also the fullfsync and checkpoint_fullfsync pragmas.
PRAGMA table_info(table-name);
This pragma returns one row for each column in the named table. Columns in the result set include the column name, data type, whether or not the column can be NULL, and the default value for the column. The "pk" column in the result set is zero for columns that are not part of the primary key, and is the index of the column in the primary key for columns that are part of the primary key.
The table named in the table_info pragma can also be a view.
PRAGMA temp_store;
PRAGMA temp_store =
0 | DEFAULT | 1 | FILE | 2 | MEMORY;
Query or change the setting of the "temp_store" parameter. When temp_store is DEFAULT (0), the compile-time C preprocessor macro SQLITE_TEMP_STORE is used to determine where temporary tables and indices are stored. When temp_store is MEMORY (2) temporary tables and indices are kept in as if they were pure in-memory databases memory. When temp_store is FILE (1) temporary tables and indices are stored in a file. The temp_store_directory pragma can be used to specify the directory containing temporary files when FILE is specified. When the temp_store setting is changed, all existing temporary tables, indices, triggers, and views are immediately deleted.
It is possible for the library compile-time C preprocessor symbol SQLITE_TEMP_STORE to override this pragma setting. The following table summarizes the interaction of the SQLITE_TEMP_STORE preprocessor macro and the temp_store pragma:
SQLITE_TEMP_STORE PRAGMA
temp_storeStorage used for
TEMP tables and indices0 any file 1 0 file 1 1 file 1 2 memory 2 0 memory 2 1 file 2 2 memory 3 any memory
PRAGMA temp_store_directory;
PRAGMA temp_store_directory = 'directory-name';
Query or change the value of the sqlite3_temp_directory global variable, which many operating-system interface backends use to determine where to store temporary tables and indices.
When the temp_store_directory setting is changed, all existing temporary tables, indices, triggers, and viewers in the database connection that issued the pragma are immediately deleted. In practice, temp_store_directory should be set immediately after the first database connection for a process is opened. If the temp_store_directory is changed for one database connection while other database connections are open in the same process, then the behavior is undefined and probably undesirable.
Changing the temp_store_directory setting is not threadsafe. Never change the temp_store_directory setting if another thread within the application is running any SQLite interface at the same time. Doing so results in undefined behavior. Changing the temp_store_directory setting writes to the sqlite3_temp_directory global variable and that global variable is not protected by a mutex.
The value directory-name should be enclosed in single quotes. To revert the directory to the default, set the directory-name to an empty string, e.g., PRAGMA temp_store_directory = ''. An error is raised if directory-name is not found or is not writable.
The default directory for temporary files depends on the OS. Some OS interfaces may choose to ignore this variable and place temporary files in some other directory different from the directory specified here. In that sense, this pragma is only advisory.
This pragma is deprecated and exists for backwards compatibility only. New applications should avoid using this pragma. Older applications should discontinue use of this pragma at the earliest opportunity. This pragma may be omitted from the build when SQLite is compiled using SQLITE_OMIT_DEPRECATED.
PRAGMA threads;
PRAGMA threads = N;
Query or change the value of the sqlite3_limit(db,SQLITE_LIMIT_WORKER_THREADS,...) limit for the current database connection. This limit sets an upper bound on the number of auxiliary threads that a prepared statement is allowed to launch to assist with a query. The default limit is 0 unless it is changed using the SQLITE_DEFAULT_WORKER_THREADS compile-time option. When the limit is zero, that means no auxiliary threads will be launched.
This pragma is a thin wrapper around the sqlite3_limit(db,SQLITE_LIMIT_WORKER_THREADS,...) interface.
PRAGMA vdbe_addoptrace = boolean;
If SQLite has been compiled with the SQLITE_DEBUG compile-time option, then the vdbe_addoptrace pragma can be used to cause a complete VDBE opcodes to be displayed as they are created during code generation. This feature is used for debugging SQLite itself. See the VDBE documentation for more information.
This pragma is intended for use when debugging SQLite itself. It is only available when the SQLITE_DEBUG compile-time option is used.
PRAGMA vdbe_debug = boolean;
If SQLite has been compiled with the SQLITE_DEBUG compile-time option, then the vdbe_debug pragma is a shorthand for three other debug-only pragmas: vdbe_addoptrace, vdbe_listing, and vdbe_trace. This feature is used for debugging SQLite itself. See the VDBE documentation for more information.
This pragma is intended for use when debugging SQLite itself. It is only available when the SQLITE_DEBUG compile-time option is used.
PRAGMA vdbe_listing = boolean;
If SQLite has been compiled with the SQLITE_DEBUG compile-time option, then the vdbe_listing pragma can be used to cause a complete listing of the virtual machine opcodes to appear on standard output as each statement is evaluated. With listing is on, the entire content of a program is printed just prior to beginning execution. The statement executes normally after the listing is printed. This feature is used for debugging SQLite itself. See the VDBE documentation for more information.
This pragma is intended for use when debugging SQLite itself. It is only available when the SQLITE_DEBUG compile-time option is used.
PRAGMA vdbe_trace = boolean;
If SQLite has been compiled with the SQLITE_DEBUG compile-time option, then the vdbe_trace pragma can be used to cause virtual machine opcodes to be printed on standard output as they are evaluated. This feature is used for debugging SQLite. See the VDBE documentation for more information.
This pragma is intended for use when debugging SQLite itself. It is only available when the SQLITE_DEBUG compile-time option is used.
PRAGMA wal_autocheckpoint;
PRAGMA wal_autocheckpoint=N;
This pragma queries or sets the write-ahead log auto-checkpoint interval. When the write-ahead log is enabled (via the journal_mode pragma) a checkpoint will be run automatically whenever the write-ahead log equals or exceeds N pages in length. Setting the auto-checkpoint size to zero or a negative value turns auto-checkpointing off.
This pragma is a wrapper around the sqlite3_wal_autocheckpoint() C interface. All automatic checkpoints are PASSIVE.
Autocheckpointing is enabled by default with an interval of 1000 or SQLITE_DEFAULT_WAL_AUTOCHECKPOINT.
PRAGMA database.wal_checkpoint;
PRAGMA database.wal_checkpoint(PASSIVE);
PRAGMA database.wal_checkpoint(FULL);
PRAGMA database.wal_checkpoint(RESTART);
If the write-ahead log is enabled (via the journal_mode pragma), this pragma causes a checkpoint operation to run on database database, or on all attached databases if database is omitted. If write-ahead log mode is disabled, this pragma is a harmless no-op.
Invoking this pragma without an argument is equivalent to calling the sqlite3_wal_checkpoint() C interface.
Invoking this pragma with an argument is equivalent to calling the sqlite3_wal_checkpoint_v2() C interface with a 3rd parameter corresponding to the argument:- PASSIVE
- Checkpoint as many frames as possible without waiting for any database readers or writers to finish. Sync the db file if all frames in the log are checkpointed. This mode is the same as calling sqlite3_wal_checkpoint(). The busy-handler callback is never invoked.
- FULL
- This mode blocks (invokes the busy-handler callback) until there is no database writer and all readers are reading from the most recent database snapshot. It then checkpoints all frames in the log file and syncs the database file. FULL blocks database writers while it is running, but not database readers.
- RESTART
- This mode works the same way as SQLITE_CHECKPOINT_FULL, except after checkpointing the log file it blocks (calls the busy-handler callback) until all readers are reading from the database file only. This ensures that the next client to write to the database file restarts the log file from the beginning. RESTART blocks database writers while it is running, but not database readers.
The wal_checkpoint pragma returns a single row with three integer columns. The first column is usually 0 but will be 1 if a RESTART or FULL checkpoint was blocked from completing, for example because another thread or process was actively using the database. In other words, the first column is 0 if the equivalent call to sqlite3_wal_checkpoint_v2() would have returned SQLITE_OK or 1 if the equivalent call would have returned SQLITE_BUSY. The second column is the number of modified pages that have been written to the write-ahead log file. The third column is the number of pages in the write-ahead log file that have been successfully moved back into the database file at the conclusion of the checkpoint. The second and third column are -1 if there is no write-ahead log, for example if this pragma is invoked on a database connection that is not in WAL mode.
PRAGMA writable_schema = boolean;
When this pragma is on, the SQLITE_MASTER tables in which database can be changed using ordinary UPDATE, INSERT, and DELETE statements. Warning: misuse of this pragma can easily result in a corrupt database file.