Use a hash table for catcache.c's CatCList objects.

Up to now, all of the "catcache list" objects within a catalog cache
were just chained together on a single dlist, requiring O(N) time to
search.  Remarkably, we've not had serious performance problems with
that so far; but we got a complaint of a bad performance regression
from v15 in a case with a large number of roles in the system, which
traced down to O(N^2) total time when we probed N catcache lists.

Replace that data structure with a hashtable having an enlargeable
number of dlists, in an exactly parallel way to the data structure
we've used for years for the plain CatCTup cache members.  The extra
cost of maintaining a hash table seems negligible, since we were
already computing a hash value for list searches.

Normally this'd be HEAD-only material, but in view of the performance
regression it seems advisable to back-patch into v16.  In the v16
version of the patch, leave the dead cc_lists field where it is and
add the new fields at the end of struct catcache, to avoid possible
ABI breakage in case any external code is looking at these structs.
(We assume no external code is actually allocating new catcache
structs.)

Per report from alex work.

Discussion: https://postgr.es/m/CAGvXd3OSMbJQwOSc-Tq-Ro1CAz=vggErdSG7pv2s6vmmTOLJSg@mail.gmail.com

src/backend/utils/cache/catcache.c

@@ -88,6 +88,8 @@ static void CatCachePrintStats(int code, Datum arg);
 #endif
 static void CatCacheRemoveCTup(CatCache *cache, CatCTup *ct);
 static void CatCacheRemoveCList(CatCache *cache, CatCList *cl);
+static void RehashCatCache(CatCache *cp);
+static void RehashCatCacheLists(CatCache *cp);
 static void CatalogCacheInitializeCache(CatCache *cache);
 static CatCTup *CatalogCacheCreateEntry(CatCache *cache,
 										HeapTuple ntp, SysScanDesc scandesc,
@@ -444,6 +446,7 @@ CatCachePrintStats(int code, Datum arg)
 	long		cc_neg_hits = 0;
 	long		cc_newloads = 0;
 	long		cc_invals = 0;
+	long		cc_nlists = 0;
 	long		cc_lsearches = 0;
 	long		cc_lhits = 0;
 
@@ -453,7 +456,7 @@ CatCachePrintStats(int code, Datum arg)
 
 		if (cache->cc_ntup == 0 && cache->cc_searches == 0)
 			continue;			/* don't print unused caches */
-		elog(DEBUG2, "catcache %s/%u: %d tup, %ld srch, %ld+%ld=%ld hits, %ld+%ld=%ld loads, %ld invals, %ld lsrch, %ld lhits",
+		elog(DEBUG2, "catcache %s/%u: %d tup, %ld srch, %ld+%ld=%ld hits, %ld+%ld=%ld loads, %ld invals, %d lists, %ld lsrch, %ld lhits",
 			 cache->cc_relname,
 			 cache->cc_indexoid,
 			 cache->cc_ntup,
@@ -465,6 +468,7 @@ CatCachePrintStats(int code, Datum arg)
 			 cache->cc_searches - cache->cc_hits - cache->cc_neg_hits - cache->cc_newloads,
 			 cache->cc_searches - cache->cc_hits - cache->cc_neg_hits,
 			 cache->cc_invals,
+			 cache->cc_nlist,
 			 cache->cc_lsearches,
 			 cache->cc_lhits);
 		cc_searches += cache->cc_searches;
@@ -472,10 +476,11 @@ CatCachePrintStats(int code, Datum arg)
 		cc_neg_hits += cache->cc_neg_hits;
 		cc_newloads += cache->cc_newloads;
 		cc_invals += cache->cc_invals;
+		cc_nlists += cache->cc_nlist;
 		cc_lsearches += cache->cc_lsearches;
 		cc_lhits += cache->cc_lhits;
 	}
-	elog(DEBUG2, "catcache totals: %d tup, %ld srch, %ld+%ld=%ld hits, %ld+%ld=%ld loads, %ld invals, %ld lsrch, %ld lhits",
+	elog(DEBUG2, "catcache totals: %d tup, %ld srch, %ld+%ld=%ld hits, %ld+%ld=%ld loads, %ld invals, %ld lists, %ld lsrch, %ld lhits",
 		 CacheHdr->ch_ntup,
 		 cc_searches,
 		 cc_hits,
@@ -485,6 +490,7 @@ CatCachePrintStats(int code, Datum arg)
 		 cc_searches - cc_hits - cc_neg_hits - cc_newloads,
 		 cc_searches - cc_hits - cc_neg_hits,
 		 cc_invals,
+		 cc_nlists,
 		 cc_lsearches,
 		 cc_lhits);
 }
@@ -573,6 +579,8 @@ CatCacheRemoveCList(CatCache *cache, CatCList *cl)
 					 cache->cc_keyno, cl->keys);
 
 	pfree(cl);
+
+	--cache->cc_nlist;
 }
 
 
@@ -611,14 +619,19 @@ CatCacheInvalidate(CatCache *cache, uint32 hashValue)
 	 * Invalidate *all* CatCLists in this cache; it's too hard to tell which
 	 * searches might still be correct, so just zap 'em all.
 	 */
-	dlist_foreach_modify(iter, &cache->cc_lists)
+	for (int i = 0; i < cache->cc_nlbuckets; i++)
 	{
-		CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
+		dlist_head *bucket = &cache->cc_lbucket[i];
 
-		if (cl->refcount > 0)
-			cl->dead = true;
-		else
-			CatCacheRemoveCList(cache, cl);
+		dlist_foreach_modify(iter, bucket)
+		{
+			CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
+
+			if (cl->refcount > 0)
+				cl->dead = true;
+			else
+				CatCacheRemoveCList(cache, cl);
+		}
 	}
 
 	/*
@@ -691,14 +704,19 @@ ResetCatalogCache(CatCache *cache)
 	int			i;
 
 	/* Remove each list in this cache, or at least mark it dead */
-	dlist_foreach_modify(iter, &cache->cc_lists)
+	for (i = 0; i < cache->cc_nlbuckets; i++)
 	{
-		CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
+		dlist_head *bucket = &cache->cc_lbucket[i];
 
-		if (cl->refcount > 0)
-			cl->dead = true;
-		else
-			CatCacheRemoveCList(cache, cl);
+		dlist_foreach_modify(iter, bucket)
+		{
+			CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
+
+			if (cl->refcount > 0)
+				cl->dead = true;
+			else
+				CatCacheRemoveCList(cache, cl);
+		}
 	}
 
 	/* Remove each tuple in this cache, or at least mark it dead */
@@ -862,6 +880,12 @@ InitCatCache(int id,
 									 MCXT_ALLOC_ZERO);
 	cp->cc_bucket = palloc0(nbuckets * sizeof(dlist_head));
 
+	/*
+	 * Many catcaches never receive any list searches.  Therefore, we don't
+	 * allocate the cc_lbuckets till we get a list search.
+	 */
+	cp->cc_lbucket = NULL;
+
 	/*
 	 * initialize the cache's relation information for the relation
 	 * corresponding to this cache, and initialize some of the new cache's
@@ -874,7 +898,9 @@ InitCatCache(int id,
 	cp->cc_relisshared = false; /* temporary */
 	cp->cc_tupdesc = (TupleDesc) NULL;
 	cp->cc_ntup = 0;
+	cp->cc_nlist = 0;
 	cp->cc_nbuckets = nbuckets;
+	cp->cc_nlbuckets = 0;
 	cp->cc_nkeys = nkeys;
 	for (i = 0; i < nkeys; ++i)
 	{
@@ -939,6 +965,44 @@ RehashCatCache(CatCache *cp)
 	cp->cc_bucket = newbucket;
 }
 
+/*
+ * Enlarge a catcache's list storage, doubling the number of buckets.
+ */
+static void
+RehashCatCacheLists(CatCache *cp)
+{
+	dlist_head *newbucket;
+	int			newnbuckets;
+	int			i;
+
+	elog(DEBUG1, "rehashing catalog cache id %d for %s; %d lists, %d buckets",
+		 cp->id, cp->cc_relname, cp->cc_nlist, cp->cc_nlbuckets);
+
+	/* Allocate a new, larger, hash table. */
+	newnbuckets = cp->cc_nlbuckets * 2;
+	newbucket = (dlist_head *) MemoryContextAllocZero(CacheMemoryContext, newnbuckets * sizeof(dlist_head));
+
+	/* Move all entries from old hash table to new. */
+	for (i = 0; i < cp->cc_nlbuckets; i++)
+	{
+		dlist_mutable_iter iter;
+
+		dlist_foreach_modify(iter, &cp->cc_lbucket[i])
+		{
+			CatCList   *cl = dlist_container(CatCList, cache_elem, iter.cur);
+			int			hashIndex = HASH_INDEX(cl->hash_value, newnbuckets);
+
+			dlist_delete(iter.cur);
+			dlist_push_head(&newbucket[hashIndex], &cl->cache_elem);
+		}
+	}
+
+	/* Switch to the new array. */
+	pfree(cp->cc_lbucket);
+	cp->cc_nlbuckets = newnbuckets;
+	cp->cc_lbucket = newbucket;
+}
+
 /*
  *		CatalogCacheInitializeCache
  *
@@ -1588,7 +1652,9 @@ SearchCatCacheList(CatCache *cache,
 	Datum		v4 = 0;			/* dummy last-column value */
 	Datum		arguments[CATCACHE_MAXKEYS];
 	uint32		lHashValue;
+	Index		lHashIndex;
 	dlist_iter	iter;
+	dlist_head *lbucket;
 	CatCList   *cl;
 	CatCTup    *ct;
 	List	   *volatile ctlist;
@@ -1602,7 +1668,7 @@ SearchCatCacheList(CatCache *cache,
 	/*
 	 * one-time startup overhead for each cache
 	 */
-	if (cache->cc_tupdesc == NULL)
+	if (unlikely(cache->cc_tupdesc == NULL))
 		CatalogCacheInitializeCache(cache);
 
 	Assert(nkeys > 0 && nkeys < cache->cc_nkeys);
@@ -1618,11 +1684,36 @@ SearchCatCacheList(CatCache *cache,
 	arguments[3] = v4;
 
 	/*
-	 * compute a hash value of the given keys for faster search.  We don't
-	 * presently divide the CatCList items into buckets, but this still lets
-	 * us skip non-matching items quickly most of the time.
+	 * If we haven't previously done a list search in this cache, create the
+	 * bucket header array; otherwise, consider whether it's time to enlarge
+	 * it.
+	 */
+	if (cache->cc_lbucket == NULL)
+	{
+		/* Arbitrary initial size --- must be a power of 2 */
+		int			nbuckets = 16;
+
+		cache->cc_lbucket = (dlist_head *)
+			MemoryContextAllocZero(CacheMemoryContext,
+								   nbuckets * sizeof(dlist_head));
+		/* Don't set cc_nlbuckets if we get OOM allocating cc_lbucket */
+		cache->cc_nlbuckets = nbuckets;
+	}
+	else
+	{
+		/*
+		 * If the hash table has become too full, enlarge the buckets array.
+		 * Quite arbitrarily, we enlarge when fill factor > 2.
+		 */
+		if (cache->cc_nlist > cache->cc_nlbuckets * 2)
+			RehashCatCacheLists(cache);
+	}
+
+	/*
+	 * Find the hash bucket in which to look for the CatCList.
 	 */
 	lHashValue = CatalogCacheComputeHashValue(cache, nkeys, v1, v2, v3, v4);
+	lHashIndex = HASH_INDEX(lHashValue, cache->cc_nlbuckets);
 
 	/*
 	 * scan the items until we find a match or exhaust our list
@@ -1630,7 +1721,8 @@ SearchCatCacheList(CatCache *cache,
 	 * Note: it's okay to use dlist_foreach here, even though we modify the
 	 * dlist within the loop, because we don't continue the loop afterwards.
 	 */
-	dlist_foreach(iter, &cache->cc_lists)
+	lbucket = &cache->cc_lbucket[lHashIndex];
+	dlist_foreach(iter, lbucket)
 	{
 		cl = dlist_container(CatCList, cache_elem, iter.cur);
 
@@ -1650,13 +1742,13 @@ SearchCatCacheList(CatCache *cache,
 			continue;
 
 		/*
-		 * We found a matching list.  Move the list to the front of the
-		 * cache's list-of-lists, to speed subsequent searches.  (We do not
+		 * We found a matching list.  Move the list to the front of the list
+		 * for its hashbucket, so as to speed subsequent searches.  (We do not
 		 * move the members to the fronts of their hashbucket lists, however,
 		 * since there's no point in that unless they are searched for
 		 * individually.)
 		 */
-		dlist_move_head(&cache->cc_lists, &cl->cache_elem);
+		dlist_move_head(lbucket, &cl->cache_elem);
 
 		/* Bump the list's refcount and return it */
 		ResourceOwnerEnlarge(CurrentResourceOwner);
@@ -1868,7 +1960,12 @@ SearchCatCacheList(CatCache *cache,
 	}
 	Assert(i == nmembers);
 
-	dlist_push_head(&cache->cc_lists, &cl->cache_elem);
+	/*
+	 * Add the CatCList to the appropriate bucket, and count it.
+	 */
+	dlist_push_head(lbucket, &cl->cache_elem);
+
+	cache->cc_nlist++;
 
 	/* Finally, bump the list's refcount and return it */
 	cl->refcount++;

src/include/utils/catcache.h

@@ -51,9 +51,11 @@ typedef struct catcache
 	CCFastEqualFN cc_fastequal[CATCACHE_MAXKEYS];	/* fast equal function for
 													 * each key */
 	int			cc_keyno[CATCACHE_MAXKEYS]; /* AttrNumber of each key */
-	dlist_head	cc_lists;		/* list of CatCList structs */
-	int			cc_ntup;		/* # of tuples currently in this cache */
 	int			cc_nkeys;		/* # of keys (1..CATCACHE_MAXKEYS) */
+	int			cc_ntup;		/* # of tuples currently in this cache */
+	int			cc_nlist;		/* # of CatCLists currently in this cache */
+	int			cc_nlbuckets;	/* # of CatCList hash buckets in this cache */
+	dlist_head *cc_lbucket;		/* hash buckets for CatCLists */
 	const char *cc_relname;		/* name of relation the tuples come from */
 	Oid			cc_reloid;		/* OID of relation the tuples come from */
 	Oid			cc_indexoid;	/* OID of index matching cache keys */