Compress GIN posting lists, for smaller index size.

GIN posting lists are now encoded using varbyte-encoding, which allows them
to fit in much smaller space than the straight ItemPointer array format used
before. The new encoding is used for both the lists stored in-line in entry
tree items, and in posting tree leaf pages.

To maintain backwards-compatibility and keep pg_upgrade working, the code
can still read old-style pages and tuples. Posting tree leaf pages in the
new format are flagged with GIN_COMPRESSED flag, to distinguish old and new
format pages. Likewise, entry tree tuples in the new format have a
GIN_ITUP_COMPRESSED flag set in a bit that was previously unused.

This patch bumps GIN_CURRENT_VERSION from 1 to 2. New indexes created with
version 9.4 will therefore have version number 2 in the metapage, while old
pg_upgraded indexes will have version 1. The code treats them the same, but
it might be come handy in the future, if we want to drop support for the
uncompressed format.

Alexander Korotkov and me. Reviewed by Tomas Vondra and Amit Langote.

1 files changed, 40 insertions, 33 deletions

diff --git a/src/backend/access/gin/ginbtree.c b/src/backend/access/gin/ginbtree.c
index 813f96a521b..c17c90fbc2e 100644
--- a/src/backend/access/gin/ginbtree.c
+++ b/src/backend/access/gin/ginbtree.c
@@ -325,9 +325,10 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 {
 	Page		page = BufferGetPage(stack->buffer);
 	XLogRecData *payloadrdata;
-	bool		fit;
+	GinPlaceToPageRC rc;
 	uint16		xlflags = 0;
 	Page		childpage = NULL;
+	Page		newlpage = NULL, newrpage = NULL;
 
 	if (GinPageIsData(page))
 		xlflags |= GIN_INSERT_ISDATA;
@@ -345,16 +346,17 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 	}
 
 	/*
-	 * Try to put the incoming tuple on the page. If it doesn't fit,
-	 * placeToPage method will return false and leave the page unmodified, and
-	 * we'll have to split the page.
+	 * Try to put the incoming tuple on the page. placeToPage will decide
+	 * if the page needs to be split.
 	 */
-	START_CRIT_SECTION();
-	fit = btree->placeToPage(btree, stack->buffer, stack->off,
-							 insertdata, updateblkno,
-							 &payloadrdata);
-	if (fit)
+	rc = btree->placeToPage(btree, stack->buffer, stack,
+							insertdata, updateblkno,
+							&payloadrdata, &newlpage, &newrpage);
+	if (rc == UNMODIFIED)
+		return true;
+	else if (rc == INSERTED)
 	{
+		/* placeToPage did START_CRIT_SECTION() */
 		MarkBufferDirty(stack->buffer);
 
 		/* An insert to an internal page finishes the split of the child. */
@@ -373,7 +375,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 
 			xlrec.node = btree->index->rd_node;
 			xlrec.blkno = BufferGetBlockNumber(stack->buffer);
-			xlrec.offset = stack->off;
 			xlrec.flags = xlflags;
 
 			rdata[0].buffer = InvalidBuffer;
@@ -415,20 +416,16 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 
 		return true;
 	}
-	else
+	else if (rc == SPLIT)
 	{
 		/* Didn't fit, have to split */
 		Buffer		rbuffer;
-		Page		newlpage;
 		BlockNumber savedRightLink;
-		Page		rpage;
 		XLogRecData rdata[2];
 		ginxlogSplit data;
 		Buffer		lbuffer = InvalidBuffer;
 		Page		newrootpg = NULL;
 
-		END_CRIT_SECTION();
-
 		rbuffer = GinNewBuffer(btree->index);
 
 		/* During index build, count the new page */
@@ -443,12 +440,9 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		savedRightLink = GinPageGetOpaque(page)->rightlink;
 
 		/*
-		 * newlpage is a pointer to memory page, it is not associated with a
-		 * buffer. stack->buffer is not touched yet.
+		 * newlpage and newrpage are pointers to memory pages, not associated
+		 * with buffers. stack->buffer is not touched yet.
 		 */
-		newlpage = btree->splitPage(btree, stack->buffer, rbuffer, stack->off,
-									insertdata, updateblkno,
-									&payloadrdata);
 
 		data.node = btree->index->rd_node;
 		data.rblkno = BufferGetBlockNumber(rbuffer);
@@ -481,8 +475,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		else
 			rdata[0].next = payloadrdata;
 
-		rpage = BufferGetPage(rbuffer);
-
 		if (stack->parent == NULL)
 		{
 			/*
@@ -508,7 +500,7 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 			data.lblkno = BufferGetBlockNumber(lbuffer);
 			data.flags |= GIN_SPLIT_ROOT;
 
-			GinPageGetOpaque(rpage)->rightlink = InvalidBlockNumber;
+			GinPageGetOpaque(newrpage)->rightlink = InvalidBlockNumber;
 			GinPageGetOpaque(newlpage)->rightlink = BufferGetBlockNumber(rbuffer);
 
 			/*
@@ -517,12 +509,12 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 			 * than overwriting the original page directly, so that we can still
 			 * abort gracefully if this fails.)
 			 */
-			newrootpg = PageGetTempPage(rpage);
-			GinInitPage(newrootpg, GinPageGetOpaque(newlpage)->flags & ~GIN_LEAF, BLCKSZ);
+			newrootpg = PageGetTempPage(newrpage);
+			GinInitPage(newrootpg, GinPageGetOpaque(newlpage)->flags & ~(GIN_LEAF | GIN_COMPRESSED), BLCKSZ);
 
 			btree->fillRoot(btree, newrootpg,
 							BufferGetBlockNumber(lbuffer), newlpage,
-							BufferGetBlockNumber(rbuffer), rpage);
+							BufferGetBlockNumber(rbuffer), newrpage);
 		}
 		else
 		{
@@ -530,7 +522,7 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 			data.rrlink = savedRightLink;
 			data.lblkno = BufferGetBlockNumber(stack->buffer);
 
-			GinPageGetOpaque(rpage)->rightlink = savedRightLink;
+			GinPageGetOpaque(newrpage)->rightlink = savedRightLink;
 			GinPageGetOpaque(newlpage)->flags |= GIN_INCOMPLETE_SPLIT;
 			GinPageGetOpaque(newlpage)->rightlink = BufferGetBlockNumber(rbuffer);
 		}
@@ -550,16 +542,24 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		START_CRIT_SECTION();
 
 		MarkBufferDirty(rbuffer);
+		MarkBufferDirty(stack->buffer);
 
+		/*
+		 * Restore the temporary copies over the real buffers. But don't free
+		 * the temporary copies yet, WAL record data points to them.
+		 */
 		if (stack->parent == NULL)
 		{
-			PageRestoreTempPage(newlpage, BufferGetPage(lbuffer));
 			MarkBufferDirty(lbuffer);
-			newlpage = newrootpg;
+			memcpy(BufferGetPage(stack->buffer), newrootpg, BLCKSZ);
+			memcpy(BufferGetPage(lbuffer), newlpage, BLCKSZ);
+			memcpy(BufferGetPage(rbuffer), newrpage, BLCKSZ);
+		}
+		else
+		{
+			memcpy(BufferGetPage(stack->buffer), newlpage, BLCKSZ);
+			memcpy(BufferGetPage(rbuffer), newrpage, BLCKSZ);
 		}
-
-		PageRestoreTempPage(newlpage, BufferGetPage(stack->buffer));
-		MarkBufferDirty(stack->buffer);
 
 		/* write WAL record */
 		if (RelationNeedsWAL(btree->index))
@@ -568,7 +568,7 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 
 			recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_SPLIT, rdata);
 			PageSetLSN(BufferGetPage(stack->buffer), recptr);
-			PageSetLSN(rpage, recptr);
+			PageSetLSN(BufferGetPage(rbuffer), recptr);
 			if (stack->parent == NULL)
 				PageSetLSN(BufferGetPage(lbuffer), recptr);
 		}
@@ -582,6 +582,11 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		if (stack->parent == NULL)
 			UnlockReleaseBuffer(lbuffer);
 
+		pfree(newlpage);
+		pfree(newrpage);
+		if (newrootpg)
+			pfree(newrootpg);
+
 		/*
 		 * If we split the root, we're done. Otherwise the split is not
 		 * complete until the downlink for the new page has been inserted to
@@ -592,6 +597,8 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		else
 			return false;
 	}
+	else
+		elog(ERROR, "unknown return code from GIN placeToPage method: %d", rc);
 }
 
 /*