Function RelationIdGetRelation

src/backend/utils/cache/relcache.c:2154–2212

* RelationIdGetRelation * * Lookup a reldesc by OID; make one if not already in cache. * * Returns NULL if no pg_class row could be found for the given relid * (suggesting we are trying to access a just-deleted relation). * Any other error is reported via elog. * * NB: caller should already have at least AccessShareLock on the * relation ID, else there are nasty race conditions. *

 *		that happens by calling RelationClose().)
 */
Relation
RelationIdGetRelation(Oid relationId)
{
	Relation	rd;

	/* Make sure we're in an xact, even if this ends up being a cache hit */
	Assert(IsTransactionState());

	/*
	 * first try to find reldesc in the cache
	 */
	RelationIdCacheLookup(relationId, rd);

	if (RelationIsValid(rd))
	{
		/* return NULL for dropped relations */
		if (rd->rd_droppedSubid != InvalidSubTransactionId)
		{
			Assert(!rd->rd_isvalid);
			return NULL;
		}

		RelationIncrementReferenceCount(rd);
		/* revalidate cache entry if necessary */
		if (!rd->rd_isvalid)
		{
			/*
			 * Indexes only have a limited number of possible schema changes,
			 * and we don't want to use the full-blown procedure because it's
			 * a headache for indexes that reload itself depends on.
			 */
			if (rd->rd_rel->relkind == RELKIND_INDEX ||
				rd->rd_rel->relkind == RELKIND_PARTITIONED_INDEX)
				RelationReloadIndexInfo(rd);
			else
				RelationClearRelation(rd, true);

			/*
			 * Normally entries need to be valid here, but before the relcache
			 * has been initialized, not enough infrastructure exists to
			 * perform pg_class lookups. The structure of such entries doesn't
			 * change, but we still want to update the rd_rel entry. So
			 * rd_isvalid = false is left in place for a later lookup.
			 */
			Assert(rd->rd_isvalid ||
				   (rd->rd_isnailed && !criticalRelcachesBuilt));
		}
		return rd;
	}

	/*
	 * no reldesc in the cache, so have RelationBuildDesc() build one and add
	 * it.
	 */
	rd = RelationBuildDesc(relationId, true);
	if (RelationIsValid(rd))
		RelationIncrementReferenceCount(rd);
	return rd;