postgresql/src/backend/catalog/pg_inherits.c

/*-------------------------------------------------------------------------
 *
 * pg_inherits.c
 *	  routines to support manipulation of the pg_inherits relation
 *
 * Note: currently, this module only contains inquiry functions; the actual
 * creation and deletion of pg_inherits entries is done in tablecmds.c.
 * Perhaps someday that code should be moved here, but it'd have to be
 * disentangled from other stuff such as pg_depend updates.
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/catalog/pg_inherits.c,v 1.3 2009/06/11 14:48:55 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "catalog/pg_class.h"
#include "catalog/pg_inherits.h"
#include "catalog/pg_inherits_fn.h"
#include "parser/parse_type.h"
#include "storage/lmgr.h"
#include "utils/fmgroids.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


/*
 * find_inheritance_children
 *
 * Returns a list containing the OIDs of all relations which
 * inherit *directly* from the relation with OID 'parentrelId'.
 *
 * The specified lock type is acquired on each child relation (but not on the
 * given rel; caller should already have locked it).  If lockmode is NoLock
 * then no locks are acquired, but caller must beware of race conditions
 * against possible DROPs of child relations.
 */
List *
find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
{
	List	   *list = NIL;
	Relation	relation;
	HeapScanDesc scan;
	ScanKeyData key[1];
	HeapTuple	inheritsTuple;
	Oid			inhrelid;

	/*
	 * Can skip the scan if pg_class shows the relation has never had a
	 * subclass.
	 */
	if (!has_subclass(parentrelId))
		return NIL;

	/*
	 * XXX might be a good idea to create an index on pg_inherits' inhparent
	 * field, so that we can use an indexscan instead of sequential scan here.
	 * However, in typical databases pg_inherits won't have enough entries to
	 * justify an indexscan...
	 */
	relation = heap_open(InheritsRelationId, AccessShareLock);
	ScanKeyInit(&key[0],
				Anum_pg_inherits_inhparent,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(parentrelId));
	scan = heap_beginscan(relation, SnapshotNow, 1, key);

	while ((inheritsTuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
	{
		inhrelid = ((Form_pg_inherits) GETSTRUCT(inheritsTuple))->inhrelid;

		if (lockmode != NoLock)
		{
			/* Get the lock to synchronize against concurrent drop */
			LockRelationOid(inhrelid, lockmode);

			/*
			 * Now that we have the lock, double-check to see if the relation
			 * really exists or not.  If not, assume it was dropped while we
			 * waited to acquire lock, and ignore it.
			 */
			if (!SearchSysCacheExists(RELOID,
									  ObjectIdGetDatum(inhrelid),
									  0, 0, 0))
			{
				/* Release useless lock */
				UnlockRelationOid(inhrelid, lockmode);
				/* And ignore this relation */
				continue;
			}
		}

		list = lappend_oid(list, inhrelid);
	}

	heap_endscan(scan);
	heap_close(relation, AccessShareLock);

	return list;
}


/*
 * find_all_inheritors -
 *		Returns a list of relation OIDs including the given rel plus
 *		all relations that inherit from it, directly or indirectly.
 *
 * The specified lock type is acquired on all child relations (but not on the
 * given rel; caller should already have locked it).  If lockmode is NoLock
 * then no locks are acquired, but caller must beware of race conditions
 * against possible DROPs of child relations.
 */
List *
find_all_inheritors(Oid parentrelId, LOCKMODE lockmode)
{
	List	   *rels_list;
	ListCell   *l;

	/*
	 * We build a list starting with the given rel and adding all direct and
	 * indirect children.  We can use a single list as both the record of
	 * already-found rels and the agenda of rels yet to be scanned for more
	 * children.  This is a bit tricky but works because the foreach() macro
	 * doesn't fetch the next list element until the bottom of the loop.
	 */
	rels_list = list_make1_oid(parentrelId);

	foreach(l, rels_list)
	{
		Oid			currentrel = lfirst_oid(l);
		List	   *currentchildren;

		/* Get the direct children of this rel */
		currentchildren = find_inheritance_children(currentrel, lockmode);

		/*
		 * Add to the queue only those children not already seen. This avoids
		 * making duplicate entries in case of multiple inheritance paths from
		 * the same parent.  (It'll also keep us from getting into an infinite
		 * loop, though theoretically there can't be any cycles in the
		 * inheritance graph anyway.)
		 */
		rels_list = list_concat_unique_oid(rels_list, currentchildren);
	}

	return rels_list;
}


/*
 * has_subclass - does this relation have any children?
 *
 * In the current implementation, has_subclass returns whether a
 * particular class *might* have a subclass. It will not return the
 * correct result if a class had a subclass which was later dropped.
 * This is because relhassubclass in pg_class is not updated when a
 * subclass is dropped, primarily because of concurrency concerns.
 *
 * Currently has_subclass is only used as an efficiency hack to skip
 * unnecessary inheritance searches, so this is OK.
 *
 * Although this doesn't actually touch pg_inherits, it seems reasonable
 * to keep it here since it's normally used with the other routines here.
 */
bool
has_subclass(Oid relationId)
{
	HeapTuple	tuple;
	bool		result;

	tuple = SearchSysCache(RELOID,
						   ObjectIdGetDatum(relationId),
						   0, 0, 0);
	if (!HeapTupleIsValid(tuple))
		elog(ERROR, "cache lookup failed for relation %u", relationId);

	result = ((Form_pg_class) GETSTRUCT(tuple))->relhassubclass;
	ReleaseSysCache(tuple);
	return result;
}


/*
 * Given two type OIDs, determine whether the first is a complex type
 * (class type) that inherits from the second.
 */
bool
typeInheritsFrom(Oid subclassTypeId, Oid superclassTypeId)
{
	bool		result = false;
	Oid			subclassRelid;
	Oid			superclassRelid;
	Relation	inhrel;
	List	   *visited,
			   *queue;
	ListCell   *queue_item;

	/* We need to work with the associated relation OIDs */
	subclassRelid = typeidTypeRelid(subclassTypeId);
	if (subclassRelid == InvalidOid)
		return false;			/* not a complex type */
	superclassRelid = typeidTypeRelid(superclassTypeId);
	if (superclassRelid == InvalidOid)
		return false;			/* not a complex type */

	/* No point in searching if the superclass has no subclasses */
	if (!has_subclass(superclassRelid))
		return false;

	/*
	 * Begin the search at the relation itself, so add its relid to the queue.
	 */
	queue = list_make1_oid(subclassRelid);
	visited = NIL;

	inhrel = heap_open(InheritsRelationId, AccessShareLock);

	/*
	 * Use queue to do a breadth-first traversal of the inheritance graph from
	 * the relid supplied up to the root.  Notice that we append to the queue
	 * inside the loop --- this is okay because the foreach() macro doesn't
	 * advance queue_item until the next loop iteration begins.
	 */
	foreach(queue_item, queue)
	{
		Oid			this_relid = lfirst_oid(queue_item);
		ScanKeyData skey;
		HeapScanDesc inhscan;
		HeapTuple	inhtup;

		/*
		 * If we've seen this relid already, skip it.  This avoids extra work
		 * in multiple-inheritance scenarios, and also protects us from an
		 * infinite loop in case there is a cycle in pg_inherits (though
		 * theoretically that shouldn't happen).
		 */
		if (list_member_oid(visited, this_relid))
			continue;

		/*
		 * Okay, this is a not-yet-seen relid. Add it to the list of
		 * already-visited OIDs, then find all the types this relid inherits
		 * from and add them to the queue.
		 */
		visited = lappend_oid(visited, this_relid);

		ScanKeyInit(&skey,
					Anum_pg_inherits_inhrelid,
					BTEqualStrategyNumber, F_OIDEQ,
					ObjectIdGetDatum(this_relid));

		inhscan = heap_beginscan(inhrel, SnapshotNow, 1, &skey);

		while ((inhtup = heap_getnext(inhscan, ForwardScanDirection)) != NULL)
		{
			Form_pg_inherits inh = (Form_pg_inherits) GETSTRUCT(inhtup);
			Oid			inhparent = inh->inhparent;

			/* If this is the target superclass, we're done */
			if (inhparent == superclassRelid)
			{
				result = true;
				break;
			}

			/* Else add to queue */
			queue = lappend_oid(queue, inhparent);
		}

		heap_endscan(inhscan);

		if (result)
			break;
	}

	/* clean up ... */
	heap_close(inhrel, AccessShareLock);

	list_free(visited);
	list_free(queue);

	return result;
}