From: Nikita Glukhov <n.gluhov@postgrespro.ru>
Date: Fri, 28 Jun 2019 23:18:39 +0300
Subject: [PATCH] Avoid full GIN index scan

---
 src/backend/access/gin/ginget.c  |  2 ++
 src/backend/access/gin/ginscan.c | 24 +++++++++++++++++++++++-
 src/backend/utils/adt/selfuncs.c | 12 +++++++++++-
 src/include/access/gin_private.h |  1 +
 4 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
index b18ae2b..317fa1f 100644
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -1891,6 +1891,8 @@ gingetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 		if (!scanGetItem(scan, iptr, &iptr, &recheck))
 			break;
 
+		recheck |= so->forcedRecheck;
+
 		if (ItemPointerIsLossyPage(&iptr))
 			tbm_add_page(tbm, ItemPointerGetBlockNumber(&iptr));
 		else
diff --git a/src/backend/access/gin/ginscan.c b/src/backend/access/gin/ginscan.c
index 74d9821..a32a6ff 100644
--- a/src/backend/access/gin/ginscan.c
+++ b/src/backend/access/gin/ginscan.c
@@ -141,7 +141,8 @@ ginFillScanKey(GinScanOpaque so, OffsetNumber attnum,
 	uint32		i;
 
 	/* Non-default search modes add one "hidden" entry to each key */
-	if (searchMode != GIN_SEARCH_MODE_DEFAULT)
+	if (searchMode != GIN_SEARCH_MODE_DEFAULT &&
+		(searchMode != GIN_SEARCH_MODE_ALL || nQueryValues))
 		nQueryValues++;
 	key->nentries = nQueryValues;
 	key->nuserentries = nUserQueryValues;
@@ -265,6 +266,7 @@ ginNewScanKey(IndexScanDesc scan)
 	GinScanOpaque so = (GinScanOpaque) scan->opaque;
 	int			i;
 	bool		hasNullQuery = false;
+	bool		hasSearchAllMode = false;
 	MemoryContext oldCtx;
 
 	/*
@@ -286,6 +288,7 @@ ginNewScanKey(IndexScanDesc scan)
 		palloc(so->allocentries * sizeof(GinScanEntry));
 
 	so->isVoidRes = false;
+	so->forcedRecheck = false;
 
 	for (i = 0; i < scan->numberOfKeys; i++)
 	{
@@ -371,6 +374,18 @@ ginNewScanKey(IndexScanDesc scan)
 					   skey->sk_argument, nQueryValues,
 					   queryValues, categories,
 					   partial_matches, extra_data);
+
+		if (searchMode == GIN_SEARCH_MODE_ALL && nQueryValues <= 0)
+		{
+			/*
+			 * Don't emit ALL key with no entries, check only whether
+			 * unconditional recheck is needed.
+			 */
+			GinScanKey	key = &so->keys[--so->nkeys];
+
+			hasSearchAllMode = true;
+			so->forcedRecheck = key->triConsistentFn(key) != GIN_TRUE;
+		}
 	}
 
 	/*
@@ -384,6 +399,13 @@ ginNewScanKey(IndexScanDesc scan)
 					   InvalidStrategy, GIN_SEARCH_MODE_EVERYTHING,
 					   (Datum) 0, 0,
 					   NULL, NULL, NULL, NULL);
+
+		/*
+		 * XXX Need to use ALL mode instead of EVERYTHING to skip NULLs if ALL
+		 * mode has been seen.
+		 */
+		if (hasSearchAllMode)
+			so->keys[so->nkeys - 1].scanEntry[0]->searchMode = GIN_SEARCH_MODE_ALL;
 	}
 
 	/*
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index d7e3f09..e54c0db 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -6258,6 +6258,16 @@ gincost_pattern(IndexOptInfo *index, int indexcol,
 		return false;
 	}
 
+	if (nentries <= 0 && searchMode == GIN_SEARCH_MODE_ALL)
+	{
+		/*
+		 * GIN does not emit scan entries for empty GIN_SEARCH_MODE_ALL keys,
+		 * and it can avoid full index scan if there are entries from other
+		 * keys, so we can skip setting of 'haveFullScan' flag.
+		 */
+		return true;
+	}
+
 	for (i = 0; i < nentries; i++)
 	{
 		/*
@@ -6641,7 +6651,7 @@ gincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 		return;
 	}
 
-	if (counts.haveFullScan || indexQuals == NIL)
+	if (counts.haveFullScan || indexQuals == NIL || counts.searchEntries <= 0)
 	{
 		/*
 		 * Full index scan will be required.  We treat this as if every key in
diff --git a/src/include/access/gin_private.h b/src/include/access/gin_private.h
index afb3e15..369b1da 100644
--- a/src/include/access/gin_private.h
+++ b/src/include/access/gin_private.h
@@ -359,6 +359,7 @@ typedef struct GinScanOpaqueData
 	MemoryContext keyCtx;		/* used to hold key and entry data */
 
 	bool		isVoidRes;		/* true if query is unsatisfiable */
+	bool		forcedRecheck;	/* recheck all returned tuples */
 } GinScanOpaqueData;
 
 typedef GinScanOpaqueData *GinScanOpaque;
-- 
2.7.4


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