1 files changed, 32 insertions, 28 deletions

diff --git a/src/backend/access/gist/gistbuild.c b/src/backend/access/gist/gistbuild.c
index 28bc5855ad9..9d3fa9c3b75 100644
--- a/src/backend/access/gist/gistbuild.c
+++ b/src/backend/access/gist/gistbuild.c
@@ -180,9 +180,7 @@ gistbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	MemoryContext oldcxt = CurrentMemoryContext;
 	int			fillfactor;
 	Oid			SortSupportFnOids[INDEX_MAX_KEYS];
-	bool		hasallsortsupports;
-	int			keyscount = IndexRelationGetNumberOfKeyAttributes(index);
-	GiSTOptions *options = NULL;
+	GiSTOptions *options = (GiSTOptions *) index->rd_options;
 
 	/*
 	 * We expect to be called exactly once for any index relation. If that's
@@ -192,9 +190,6 @@ gistbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 		elog(ERROR, "index \"%s\" already contains data",
 			 RelationGetRelationName(index));
 
-	if (index->rd_options)
-		options = (GiSTOptions *) index->rd_options;
-
 	buildstate.indexrel = index;
 	buildstate.heaprel = heap;
 	buildstate.sortstate = NULL;
@@ -208,33 +203,17 @@ gistbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	buildstate.giststate->tempCxt = createTempGistContext();
 
 	/*
-	 * Choose build strategy. If all keys support sorting, do that. Otherwise
-	 * the default strategy is switch to buffering mode when the index grows
-	 * too large to fit in cache.
+	 * Choose build strategy.  First check whether the user specified to use
+	 * buffering mode.  (The use-case for that in the field is somewhat
+	 * questionable perhaps, but it's important for testing purposes.)
 	 */
-	hasallsortsupports = true;
-	for (int i = 0; i < keyscount; i++)
-	{
-		SortSupportFnOids[i] = index_getprocid(index, i + 1,
-											   GIST_SORTSUPPORT_PROC);
-		if (!OidIsValid(SortSupportFnOids[i]))
-		{
-			hasallsortsupports = false;
-			break;
-		}
-	}
-
-	if (hasallsortsupports)
-	{
-		buildstate.buildMode = GIST_SORTED_BUILD;
-	}
-	else if (options)
+	if (options)
 	{
 		if (options->buffering_mode == GIST_OPTION_BUFFERING_ON)
 			buildstate.buildMode = GIST_BUFFERING_STATS;
 		else if (options->buffering_mode == GIST_OPTION_BUFFERING_OFF)
 			buildstate.buildMode = GIST_BUFFERING_DISABLED;
-		else
+		else					/* must be "auto" */
 			buildstate.buildMode = GIST_BUFFERING_AUTO;
 	}
 	else
@@ -243,6 +222,28 @@ gistbuild(Relation heap, Relation index, IndexInfo *indexInfo)
 	}
 
 	/*
+	 * Unless buffering mode was forced, see if we can use sorting instead.
+	 */
+	if (buildstate.buildMode != GIST_BUFFERING_STATS)
+	{
+		bool		hasallsortsupports = true;
+		int			keyscount = IndexRelationGetNumberOfKeyAttributes(index);
+
+		for (int i = 0; i < keyscount; i++)
+		{
+			SortSupportFnOids[i] = index_getprocid(index, i + 1,
+												   GIST_SORTSUPPORT_PROC);
+			if (!OidIsValid(SortSupportFnOids[i]))
+			{
+				hasallsortsupports = false;
+				break;
+			}
+		}
+		if (hasallsortsupports)
+			buildstate.buildMode = GIST_SORTED_BUILD;
+	}
+
+	/*
 	 * Calculate target amount of free space to leave on pages.
 	 */
 	fillfactor = options ? options->fillfactor : GIST_DEFAULT_FILLFACTOR;
@@ -852,7 +853,10 @@ gistBuildCallback(Relation index,
 	 * and switch to buffering mode if it has.
 	 *
 	 * To avoid excessive calls to smgrnblocks(), only check this every
-	 * BUFFERING_MODE_SWITCH_CHECK_STEP index tuples
+	 * BUFFERING_MODE_SWITCH_CHECK_STEP index tuples.
+	 *
+	 * In 'stats' state, switch as soon as we have seen enough tuples to have
+	 * some idea of the average tuple size.
 	 */
 	if ((buildstate->buildMode == GIST_BUFFERING_AUTO &&
 		 buildstate->indtuples % BUFFERING_MODE_SWITCH_CHECK_STEP == 0 &&