Determine If An SQL Statement Writes The Database

Small. Fast. Reliable.
Choose any three.

## SQLite C Interface

## Determine If An SQL Statement Writes The Database

>     int sqlite3_stmt_readonly(sqlite3_stmt *pStmt);

The sqlite3_stmt_readonly(X) interface returns true (non-zero) if and only if the prepared statement X makes no direct changes to the content of the database file.

Note that application-defined SQL functions or virtual tables might change the database indirectly as a side effect. For example, if an application defines a function "eval()" that calls sqlite3_exec(), then the following SQL statement would change the database file through side-effects:

>     SELECT eval('DELETE FROM t1') FROM t2;

But because the SELECT statement does not change the database file directly, sqlite3_stmt_readonly() would still return true.

Transaction control statements such as BEGIN, COMMIT, ROLLBACK, SAVEPOINT, and RELEASE cause sqlite3_stmt_readonly() to return true, since the statements themselves do not actually modify the database but rather they control the timing of when other statements modify the database. The ATTACH and DETACH statements also cause sqlite3_stmt_readonly() to return true since, while those statements change the configuration of a database connection, they do not make changes to the content of the database files on disk. The sqlite3_stmt_readonly() interface returns true for BEGIN since BEGIN merely sets internal flags, but the BEGIN IMMEDIATE and BEGIN EXCLUSIVE commands do touch the database and so sqlite3_stmt_readonly() returns false for those commands.

This routine returns false if there is any possibility that the statement might change the database file. A false return does not guarantee that the statement will change the database file. For example, an UPDATE statement might have a WHERE clause that makes it a no-op, but the sqlite3_stmt_readonly() result would still be false. Similarly, a CREATE TABLE IF NOT EXISTS statement is a read-only no-op if the table already exists, but sqlite3_stmt_readonly() still returns false for such a statement.

If prepared statement X is an EXPLAIN or EXPLAIN QUERY PLAN statement, then sqlite3_stmt_readonly(X) returns the same value as if the EXPLAIN or EXPLAIN QUERY PLAN prefix were omitted.

See also lists of Objects, Constants, and Functions.