#define SQLITE_VTAB_CONSTRAINT_SUPPORT 1
If X is non-zero, then the virtual table implementation guarantees that if xUpdate returns SQLITE_CONSTRAINT, it will do so before any modifications to internal or persistent data structures have been made. If the ON CONFLICT mode is ABORT, FAIL, IGNORE or ROLLBACK, SQLite is able to roll back a statement or database transaction, and abandon or continue processing the current SQL statement as appropriate. If the ON CONFLICT mode is REPLACE and the xUpdate method returns SQLITE_CONSTRAINT, SQLite handles this as if the ON CONFLICT mode had been ABORT.
Virtual table implementations that are required to handle OR REPLACE must do so within the xUpdate method. If a call to the sqlite3_vtab_on_conflict() function indicates that the current ON CONFLICT policy is REPLACE, the virtual table implementation should silently replace the appropriate rows within the xUpdate callback and return SQLITE_OK. Or, if this is not possible, it may return SQLITE_CONSTRAINT, in which case SQLite falls back to OR ABORT constraint handling.
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