Function SerializationNeededForRead

src/backend/storage/lmgr/predicate.c:513–551

* When a public interface method is called for a read, this is the test to * see if we should do a quick return. * * Note: this function has side-effects! If this transaction has been flagged * as RO-safe since the last call, we release all predicate locks and reset * MySerializableXact. That makes subsequent calls to return quickly. * * This is marked as 'inline' to eliminate the function

 * common case that serialization is not needed.
 */
static inline bool
SerializationNeededForRead(Relation relation, Snapshot snapshot)
{
	/* Nothing to do if this is not a serializable transaction */
	if (MySerializableXact == InvalidSerializableXact)
		return false;

	/*
	 * Don't acquire locks or conflict when scanning with a special snapshot.
	 * This excludes things like CLUSTER and REINDEX. They use the wholesale
	 * functions TransferPredicateLocksToHeapRelation() and
	 * CheckTableForSerializableConflictIn() to participate in serialization,
	 * but the scans involved don't need serialization.
	 */
	if (!IsMVCCSnapshot(snapshot))
		return false;

	/*
	 * Check if we have just become "RO-safe". If we have, immediately release
	 * all locks as they're not needed anymore. This also resets
	 * MySerializableXact, so that subsequent calls to this function can exit
	 * quickly.
	 *
	 * A transaction is flagged as RO_SAFE if all concurrent R/W transactions
	 * commit without having conflicts out to an earlier snapshot, thus
	 * ensuring that no conflicts are possible for this transaction.
	 */
	if (SxactIsROSafe(MySerializableXact))
	{
		ReleasePredicateLocks(false, true);
		return false;
	}

	/* Check if the relation doesn't participate in predicate locking */
	if (!PredicateLockingNeededForRelation(relation))
		return false;

	return true;				/* no excuse to skip predicate locking */
}

/*
 * Like SerializationNeededForRead(), but called on writes.