From 013ebc0a7b7ea9c1b1ab7a3d4dd75ea121ea8ba7 Mon Sep 17 00:00:00 2001
From: Teodor Sigaev <teodor@sigaev.ru>
Date: Wed, 9 Sep 2015 18:43:37 +0300
Subject: Microvacuum for GIST

Mark index tuple as dead if it's pointed by kill_prior_tuple during
ordinary (search) scan and remove it during insert process if there is no
enough space for new tuple to insert. This improves select performance
because index will not return tuple marked as dead and improves insert
performance because it reduces number of page split.

Anastasia Lubennikova <a.lubennikova@postgrespro.ru> with
 minor editorialization by me
---
 src/backend/access/gist/gist.c | 82 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)

(limited to 'src/backend/access/gist/gist.c')

diff --git a/src/backend/access/gist/gist.c b/src/backend/access/gist/gist.c
index 0e499598a4..4edc5a75f2 100644
--- a/src/backend/access/gist/gist.c
+++ b/src/backend/access/gist/gist.c
@@ -36,6 +36,7 @@ static bool gistinserttuples(GISTInsertState *state, GISTInsertStack *stack,
 				 bool unlockbuf, bool unlockleftchild);
 static void gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
 				GISTSTATE *giststate, List *splitinfo, bool releasebuf);
+static void gistvacuumpage(Relation rel, Page page, Buffer buffer);
 
 
 #define ROTATEDIST(d) do { \
@@ -209,6 +210,17 @@ gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
 	 * because the tuple vector passed to gistSplit won't include this tuple.
 	 */
 	is_split = gistnospace(page, itup, ntup, oldoffnum, freespace);
+
+	/*
+	 * If leaf page is full, try at first to delete dead tuples. And then
+	 * check again.
+	 */
+	if (is_split && GistPageIsLeaf(page) && GistPageHasGarbage(page))
+	{
+		gistvacuumpage(rel, page, buffer);
+		is_split = gistnospace(page, itup, ntup, oldoffnum, freespace);
+	}
+
 	if (is_split)
 	{
 		/* no space for insertion */
@@ -1440,3 +1452,73 @@ freeGISTstate(GISTSTATE *giststate)
 	/* It's sufficient to delete the scanCxt */
 	MemoryContextDelete(giststate->scanCxt);
 }
+
+/*
+ * gistvacuumpage() -- try to remove LP_DEAD items from the given page.
+ * Function assumes that buffer is exclusively locked.
+ */
+static void
+gistvacuumpage(Relation rel, Page page, Buffer buffer)
+{
+	OffsetNumber deletable[MaxIndexTuplesPerPage];
+	int			 ndeletable = 0;
+	OffsetNumber offnum, maxoff;
+
+	Assert(GistPageIsLeaf(page));
+
+	/*
+	 * Scan over all items to see which ones need to be deleted according to
+	 * LP_DEAD flags.
+	 */
+	maxoff = PageGetMaxOffsetNumber(page);
+	for (offnum = FirstOffsetNumber;
+		 offnum <= maxoff;
+		 offnum = OffsetNumberNext(offnum))
+	{
+		ItemId		itemId = PageGetItemId(page, offnum);
+
+		if (ItemIdIsDead(itemId))
+			deletable[ndeletable++] = offnum;
+	}
+
+	if (ndeletable > 0)
+	{
+		START_CRIT_SECTION();
+
+		PageIndexMultiDelete(page, deletable, ndeletable);
+
+		/*
+		 * Mark the page as not containing any LP_DEAD items.  This is not
+		 * certainly true (there might be some that have recently been marked,
+		 * but weren't included in our target-item list), but it will almost
+		 * always be true and it doesn't seem worth an additional page scan to
+		 * check it. Remember that F_HAS_GARBAGE is only a hint anyway.
+		 */
+		GistClearPageHasGarbage(page);
+
+		MarkBufferDirty(buffer);
+
+		/* XLOG stuff */
+		if (RelationNeedsWAL(rel))
+		{
+			XLogRecPtr	recptr;
+
+			recptr = gistXLogUpdate(rel->rd_node, buffer,
+									deletable, ndeletable,
+									NULL, 0, InvalidBuffer);
+
+			PageSetLSN(page, recptr);
+		}
+		else
+			PageSetLSN(page, gistGetFakeLSN(rel));
+
+		END_CRIT_SECTION();
+	}
+
+	/*
+	 * Note: if we didn't find any LP_DEAD items, then the page's
+	 * F_HAS_GARBAGE hint bit is falsely set.  We do not bother expending a
+	 * separate write to clear it, however.  We will clear it when we split
+	 * the page.
+	 */
+}
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