int sqlite3_snapshot_open( sqlite3 *db, const char *zSchema, sqlite3_snapshot *pSnapshot );
The sqlite3_snapshot_open(D,S,P) interface either starts a new read transaction or upgrades an existing one for schema S of database connection D such that the read transaction refers to historical snapshot P, rather than the most recent change to the database. The sqlite3_snapshot_open() interface returns SQLITE_OK on success or an appropriate error code if it fails.
In order to succeed, the database connection must not be in autocommit mode when sqlite3_snapshot_open(D,S,P) is called. If there is already a read transaction open on schema S, then the database handle must have no active statements (SELECT statements that have been passed to sqlite3_step() but not sqlite3_reset() or sqlite3_finalize()). SQLITE_ERROR is returned if either of these conditions is violated, or if schema S does not exist, or if the snapshot object is invalid.
A call to sqlite3_snapshot_open() will fail to open if the specified snapshot has been overwritten by a checkpoint. In this case SQLITE_ERROR_SNAPSHOT is returned.
If there is already a read transaction open when this function is invoked, then the same read transaction remains open (on the same database snapshot) if SQLITE_ERROR, SQLITE_BUSY or SQLITE_ERROR_SNAPSHOT is returned. If another error code - for example SQLITE_PROTOCOL or an SQLITE_IOERR error code - is returned, then the final state of the read transaction is undefined. If SQLITE_OK is returned, then the read transaction is now open on database snapshot P.
A call to sqlite3_snapshot_open(D,S,P) will fail if the database connection D does not know that the database file for schema S is in WAL mode. A database connection might not know that the database file is in WAL mode if there has been no prior I/O on that database connection, or if the database entered WAL mode after the most recent I/O on the database connection. (Hint: Run "PRAGMA application_id" against a newly opened database connection in order to make it ready to use snapshots.)
The sqlite3_snapshot_open() interface is only available when the SQLITE_ENABLE_SNAPSHOT compile-time option is used.
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