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The PRAGMA statement is a special SQL statement used to modify the operation of the SQLite library or to query the 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. Use with caution!
- 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 be applied to the library during the sqlite3_prepare() call.
- The pragma command is unlikely to be compatible with any other SQL engine.

The available pragmas fall into four basic categories:

- Pragmas used to modify the operation of the SQLite library in some manner, or to query for the current mode of operation.
- Pragmas used to query the schema of the current database.
- Pragmas used to query or modify the databases two version values, the schema-version and the user-version.
- Pragmas used to debug the library and verify that database files are not corrupted.

# PRAGMA command syntax

sql-statement ::= | PRAGMA name [= value] | PRAGMA function(arg)

The pragmas that take an integer value also accept symbolic names. The strings "on", "true", and "yes" are equivalent to 1. The strings "off", "false", and "no" are equivalent to 0. These strings are case- insensitive, and do not require quotes. An unrecognized string will be treated as 1, and will not generate an error. When the value is returned it is as an integer.

# Pragmas to modify library operation

- PRAGMA auto_vacuum;
  PRAGMA auto_vacuum = 0 | none | 1 | full | 2 | incremental;

  Query or set the auto-vacuum flag in the database.

  Normally, (that is to say when auto_vacuum is 0 or "none") when a transaction that deletes data from a database is committed, the database file remains the same size. Unused database file pages are added to a "freelist" are reused for subsequent inserts. The database file does not shrink. In this mode the VACUUM command can be used to reclaim unused space.

  When the auto-vacuum flag is 1 (full), the freelist pages are moved to the end of the file and the file is truncated to remove the freelist pages at every 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 referer. 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 (incremental) then the additional information needed to do autovacuuming is stored in the database file but autovacuuming 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 vacuum to occur.

  The database connection can be changed between full and incremental autovacuum mode at will. However, the connection cannot be changed in and out of the "none" mode after any table has been created in the database.

- PRAGMA cache_size;
  PRAGMA cache_size = Number-of-pages;

  Query or change the maximum number of database disk pages that SQLite will hold in memory at once. Each page uses about 1.5K of memory. The default cache size is 2000. If you are doing UPDATEs or DELETEs that change many rows of a database and you do not mind if SQLite uses more memory, you can increase the cache size for a possible speed improvement.

  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. Use the default_cache_size pragma to check the cache size permanently.

- PRAGMA case_sensitive_like;
  PRAGMA case_sensitive_like = 0 | 1;

  The default behavior of the LIKE operator is to ignore case for latin1 characters. Hence, by default 'a' LIKE 'A' is true. The case_sensitive_like pragma can be turned on to change this behavior. When case_sensitive_like is enabled, 'a' LIKE 'A' is false but 'a' LIKE 'a' is still true.

- PRAGMA count_changes;
  PRAGMA count_changes = 0 | 1;

  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.

- PRAGMA default_cache_size;
  PRAGMA default_cache_size = Number-of-pages;

  Query or change the maximum number of database disk pages that SQLite will hold in memory at once. Each page uses 1K on disk and about 1.5K in memory. This pragma works like the cache_size pragma with the additional feature that it changes the cache size persistently. With this pragma, you can set the cache size once and that setting is retained and reused every time you reopen the database.

- PRAGMA empty_result_callbacks;
  PRAGMA empty_result_callbacks = 0 | 1;

  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() call 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.

- 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 and subsequent forms of this pragma are only useful if the main database has not already been created. In this case the pragma sets 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". If the second and subsequent forms are used after the database file has already been created, they have no effect and are 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.

- PRAGMA full_column_names;
  PRAGMA full_column_names = 0 | 1;

  Query or change the full-column-names flag. This flag affects the way SQLite names columns of data returned by SELECT statements when the expression for the column is a table-column name or the wildcard "*". Normally, such result columns are named <table-name/alias><column-name> if the SELECT statement joins two or more tables together, or simply <column-name> if the SELECT statement queries a single table. When the full-column-names flag is set, such columns are always named <table-name/alias> <column-name> regardless of whether or not a join is performed.

  If both the short-column-names and full-column-names are set, then the behaviour associated with the full-column-names flag is exhibited.

- PRAGMA fullfsync
  PRAGMA fullfsync = 0 | 1;

  Query or change the fullfsync flag. This flag affects determines whether or not the F_FULLFSYNC syncing method is used on systems that support it. The default value is off. As of this writing (2006-02-10) only Mac OS X supports F_FULLFSYNC.

- 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, then the entire freelist is cleared.

  As of version 3.4.0 (the first version that supports incremental_vacuum) this feature is still experimental. Possible future changes include enhancing incremental vacuum to do defragmentation and node repacking just as the full-blown VACUUM command does. And incremental vacuum may be promoted from a pragma to a separate SQL command, or perhaps some variation on the VACUUM command. Programmers are cautioned to not become enamored with the current syntax or functionality as it is likely to change.

- PRAGMA legacy_file_format;
  PRAGMA legacy_file_format = ON | OFF

  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 older versions of SQLite.

  When the 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.

  This flag only affects newly created databases. It has no effect on databases that already exist.

- 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), 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 be released until the next time the database file is accessed.

  There are two reasons to set the locking-mode to EXCLUSIVE. One is if the application actually wants to prevent other processes from accessing the database file. The other is that a small number of filesystem operations are saved by optimizations enabled in this mode. This may be significant in embedded environments.

  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 preceeds 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) always uses exclusive locking mode. The locking mode of temp cannot be changed. All other databases use the normal locking mode by default and are affected by this pragma.

- PRAGMA page_size;
  PRAGMA page_size = bytes;

  Query or set the page-size of the database. The page-size may only be set if the database has not yet been created. The page size must be a power of two greater than or equal to 512 and less than or equal to 8192. The upper limit may be modified by setting the value of macro SQLITE_MAX_PAGE_SIZE during compilation. The maximum upper bound is 32768.

- 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 read_uncommitted;
  PRAGMA read_uncommitted = 0 | 1;

  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 and is only available between connections running the same thread. Cache sharing is off by default.

- PRAGMA short_column_names;
  PRAGMA short_column_names = 0 | 1;

  Query or change the short-column-names flag. This flag affects the way SQLite names columns of data returned by SELECT statements when the expression for the column is a table-column name or the wildcard "*". Normally, such result columns are named <table-name/alias>lt;column-name> if the SELECT statement joins two or more tables together, or simply <column-name> if the SELECT statement queries a single table. When the short-column-names flag is set, such columns are always named <column-name> regardless of whether or not a join is performed.

  If both the short-column-names and full-column-names are set, then the behaviour associated with the full-column-names flag is exhibited.

- PRAGMA synchronous;
  PRAGMA synchronous = FULL; (2)
  PRAGMA synchronous = NORMAL; (1)
  PRAGMA synchronous = OFF; (0)

  Query or change the setting of the "synchronous" flag. The first (query) form will return the setting as an integer. When synchronous is FULL (2), the SQLite database engine will pause at critical moments to make sure that data has actually been written to the disk surface before continuing. This ensures that if the operating system crashes or if there is a power failure, the database will be uncorrupted after rebooting. FULL synchronous is very safe, but it is also slow. When synchronous is NORMAL, the SQLite database engine will still pause 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 pausing 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 SQLite version 2, the default value is NORMAL. For version 3, the default was changed to FULL.

- PRAGMA temp_store;
  PRAGMA temp_store = DEFAULT; (0)
  PRAGMA temp_store = FILE; (1)
  PRAGMA temp_store = MEMORY; (2)

  Query or change the setting of the "temp_store" parameter. When temp_store is DEFAULT (0), the compile-time C preprocessor macro 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 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 this file. 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 TEMP_STORE to override this pragma setting. The following table summarizes the interaction of the TEMP_STORE preprocessor macro and the temp_store pragma:

  > TEMP_STORE | PRAGMA temp_store | Storage used for TEMP tables and indices
  > --- | --- | ---
  > 0 | 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 setting of the "temp_store_directory" - the directory where files used for storing temporary tables and indices are kept. This setting lasts for the duration of the current connection only and resets to its default value for each new connection opened.

  When the temp_store_directory setting is changed, all existing temporary tables, indices, triggers, and viewers are immediately deleted. In practice, temp_store_directory should be set immediately after the database is opened.

  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. For Unix/Linux/OSX, the default is the is the first writable directory found in the list of: /var/tmp, /usr/tmp, /tmp, and current-directory. For Windows NT, the default directory is determined by Windows, generally C:\Documents and Settings\user-name\Local Settings\Temp\. Temporary files created by SQLite are unlinked immediately after opening, so that the operating system can automatically delete the files when the SQLite process exits. Thus, temporary files are not normally visible through ls or dir commands.

# Pragmas to query the database schema

- PRAGMA database_list;

  For each open database, invoke the callback function once with information about that database. Arguments include the index and the name the database was attached with. The first row will be for the main database. The second row will be for the database used to store temporary tables.

- PRAGMA foreign_key_list(table-name);

  For each foreign key that references a column in the argument table, invoke the callback function with information about that foreign key. The callback function will be invoked once for each column in each foreign key.

- PRAGMA [database].freelist_count;

  Return the number of unused pages in the database file. Running a "PRAGMA incremental_vaccum(N);" command with a large value of N will shrink the database file by this number of pages.

- PRAGMA index_info(index-name);

  For each column that the named index references, invoke the callback function once with information about that column, including the column name, and the column number.

- PRAGMA index_list(table-name);

  For each index on the named table, invoke the callback function once with information about that index. Arguments include the index name and a flag to indicate whether or not the index must be unique.

- PRAGMA table_info(table-name);

  For each column in the named table, invoke the callback function once with information about that column, including the column name, data type, whether or not the column can be NULL, and the default value for the column.

# Pragmas to query/modify version values

- PRAGMA [database.]schema_version;
  PRAGMA [database.]schema_version = integer ;
  PRAGMA [database.]user_version;
  PRAGMA [database.]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. Both the schema-version and the user-version are 32-bit signed integers stored in the database header.

  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.

# Pragmas to debug the library

- PRAGMA integrity_check;
  PRAGMA integrity_check(integer)

  The command does an integrity check of the entire database. It looks for out-of-order records, missing pages, malformed records, and corrupt indices. If any problems are found, then strings are returned (as multiple rows with a single column per row) which describe the problems. At most integer errors will be reported before the analysis quits. The default value for integer is 100. If no errors are found, a single row with the value "ok" is returned.

- PRAGMA parser_trace = ON; (1)
  PRAGMA parser_trace = OFF; (0)

  Turn tracing of the SQL parser inside of the SQLite library on and off. This is used for debugging. This only works if the library is compiled without the NDEBUG macro.

- PRAGMA vdbe_trace = ON; (1)
  PRAGMA vdbe_trace = OFF; (0)

  Turn tracing of the virtual database engine inside of the SQLite library on and off. This is used for debugging. See the VDBE documentation for more information.

- PRAGMA vdbe_listing = ON; (1)
  PRAGMA vdbe_listing = OFF; (0)

  Turn listings of virtual machine programs on and off. With listing is on, the entire content of a program is printed just prior to beginning execution. This is like automatically executing an EXPLAIN prior to each statement. The statement executes normally after the listing is printed. This is used for debugging. See the VDBE documentation for more information.

This page last modified 2007/12/20 20:49:53 UTC