int sqlite3_snapshot_open( sqlite3 *db, const char *zSchema, sqlite3_snapshot *pSnapshot );
Important: This interface is experimental and is subject to change without notice.
The sqlite3_snapshot_open(D,S,P) interface starts a read transaction 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, a call to sqlite3_snapshot_open(D,S,P) must be the first operation following the BEGIN that takes the schema S out of autocommit mode. In other words, schema S must not currently be in a transaction for sqlite3_snapshot_open(D,S,P) to work, but the database connection D must be out of autocommit mode. A snapshot will fail to open if it has been overwritten by a checkpoint. 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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