pg_inherits.c

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/*-------------------------------------------------------------------------
 *
 * 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) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/catalog/pg_inherits.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "catalog/indexing.h"
#include "catalog/pg_inherits.h"
#include "catalog/pg_inherits_fn.h"
#include "parser/parse_type.h"
#include "storage/lmgr.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/memutils.h"
#include "utils/syscache.h"
#include "utils/tqual.h"

/*
 * Entry of a hash table used in find_all_inheritors. See below.
 */
typedef struct SeenRelsEntry
{
    Oid         rel_id;         /* relation oid */
    ListCell   *numparents_cell;    /* corresponding list cell */
} SeenRelsEntry;

/*
 * 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;
    SysScanDesc scan;
    ScanKeyData key[1];
    HeapTuple   inheritsTuple;
    Oid         inhrelid;
    Oid        *oidarr;
    int         maxoids,
                numoids,
                i;

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

    /*
     * Scan pg_inherits and build a working array of subclass OIDs.
     */
    maxoids = 32;
    oidarr = (Oid *) palloc(maxoids * sizeof(Oid));
    numoids = 0;

    relation = heap_open(InheritsRelationId, AccessShareLock);

    ScanKeyInit(&key[0],
                Anum_pg_inherits_inhparent,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(parentrelId));

    scan = systable_beginscan(relation, InheritsParentIndexId, true,
                              NULL, 1, key);

    while ((inheritsTuple = systable_getnext(scan)) != NULL)
    {
        inhrelid = ((Form_pg_inherits) GETSTRUCT(inheritsTuple))->inhrelid;
        if (numoids >= maxoids)
        {
            maxoids *= 2;
            oidarr = (Oid *) repalloc(oidarr, maxoids * sizeof(Oid));
        }
        oidarr[numoids++] = inhrelid;
    }

    systable_endscan(scan);

    heap_close(relation, AccessShareLock);

    /*
     * If we found more than one child, sort them by OID.  This ensures
     * reasonably consistent behavior regardless of the vagaries of an
     * indexscan.  This is important since we need to be sure all backends
     * lock children in the same order to avoid needless deadlocks.
     */
    if (numoids > 1)
        qsort(oidarr, numoids, sizeof(Oid), oid_cmp);

    /*
     * Acquire locks and build the result list.
     */
    for (i = 0; i < numoids; i++)
    {
        inhrelid = oidarr[i];

        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 (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(inhrelid)))
            {
                /* Release useless lock */
                UnlockRelationOid(inhrelid, lockmode);
                /* And ignore this relation */
                continue;
            }
        }

        list = lappend_oid(list, inhrelid);
    }

    pfree(oidarr);

    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.
 *      Optionally, it also returns the number of parents found for
 *      each such relation within the inheritance tree rooted at the
 *      given rel.
 *
 * 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 **numparents)
{
    /* hash table for O(1) rel_oid -> rel_numparents cell lookup */
    HTAB       *seen_rels;
    HASHCTL     ctl;
    List       *rels_list,
               *rel_numparents;
    ListCell   *l;

    memset(&ctl, 0, sizeof(ctl));
    ctl.keysize = sizeof(Oid);
    ctl.entrysize = sizeof(SeenRelsEntry);
    ctl.hcxt = CurrentMemoryContext;

    seen_rels = hash_create("find_all_inheritors temporary table",
                            32, /* start small and extend */
                            &ctl,
                            HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    /*
     * 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);
    rel_numparents = list_make1_int(0);

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

        /* 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.)
         */
        foreach(lc, currentchildren)
        {
            Oid         child_oid = lfirst_oid(lc);
            bool        found;
            SeenRelsEntry *hash_entry;

            hash_entry = hash_search(seen_rels, &child_oid, HASH_ENTER, &found);
            if (found)
            {
                /* if the rel is already there, bump number-of-parents counter */
                lfirst_int(hash_entry->numparents_cell)++;
            }
            else
            {
                /* if it's not there, add it. expect 1 parent, initially. */
                rels_list = lappend_oid(rels_list, child_oid);
                rel_numparents = lappend_int(rel_numparents, 1);
                hash_entry->numparents_cell = rel_numparents->tail;
            }
        }
    }

    if (numparents)
        *numparents = rel_numparents;
    else
        list_free(rel_numparents);

    hash_destroy(seen_rels);

    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 immediately
 * 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.  Note that ANALYZE
 * on a childless table will clean up the obsolete relhassubclass flag.
 *
 * 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 = SearchSysCache1(RELOID, ObjectIdGetDatum(relationId));
    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;
        SysScanDesc 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 = systable_beginscan(inhrel, InheritsRelidSeqnoIndexId, true,
                                     NULL, 1, &skey);

        while ((inhtup = systable_getnext(inhscan)) != 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);
        }

        systable_endscan(inhscan);

        if (result)
            break;
    }

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

    list_free(visited);
    list_free(queue);

    return result;
}