Committed calls f after committed or immediate if not in tx.

Exec calls db.ExecContext.

IsInTx checks is context inside RunInTx.

Iter calls mysql.IterContext.

Middleware injects db into request's context.

NewContext creates new context.

Prepare calls db.PrepareContext.

Query calls db.QueryContext.

QueryRow calls db.QueryRowContext.

RunInTx calls RunInTxOptions with default options.

RunInTxOptions starts sql tx if not started.

Queryer interface.