Rewrite the way GIN posting lists are packed on a page, to reduce WAL volume.

Inserting (in retail) into the new 9.4 format GIN posting tree created much
larger WAL records than in 9.3. The previous strategy to WAL logging was
basically to log the whole page on each change, with the exception of
completely unmodified segments up to the first modified one. That was not
too bad when appending to the end of the page, as only the last segment had
to be WAL-logged, but per Fujii Masao's testing, even that produced 2x the
WAL volume that 9.3 did.

The new strategy is to keep track of changes to the posting lists in a more
fine-grained fashion, and also make the repacking" code smarter to avoid
decoding and re-encoding segments unnecessarily.

1 files changed, 59 insertions, 6 deletions

diff --git a/src/backend/access/rmgrdesc/gindesc.c b/src/backend/access/rmgrdesc/gindesc.c
index 1983bcc94c..aa60c8db65 100644
--- a/src/backend/access/rmgrdesc/gindesc.c
+++ b/src/backend/access/rmgrdesc/gindesc.c
@@ -25,6 +25,57 @@ desc_node(StringInfo buf, RelFileNode node, BlockNumber blkno)
 					 node.spcNode, node.dbNode, node.relNode, blkno);
 }
 
+static void
+desc_recompress_leaf(StringInfo buf, ginxlogRecompressDataLeaf *insertData)
+{
+	int			i;
+	char	   *walbuf = ((char *) insertData) + sizeof(ginxlogRecompressDataLeaf);
+
+	appendStringInfo(buf, " %d segments:", (int) insertData->nactions);
+
+	for (i = 0; i < insertData->nactions; i++)
+	{
+		uint8		a_segno = *((uint8 *) (walbuf++));
+		uint8		a_action = *((uint8 *) (walbuf++));
+		uint16		nitems = 0;
+		int			newsegsize = 0;
+
+		if (a_action == GIN_SEGMENT_INSERT ||
+			a_action == GIN_SEGMENT_REPLACE)
+		{
+			newsegsize = SizeOfGinPostingList((GinPostingList *) walbuf);
+			walbuf += SHORTALIGN(newsegsize);
+		}
+
+		if (a_action == GIN_SEGMENT_ADDITEMS)
+		{
+			memcpy(&nitems, walbuf, sizeof(uint16));
+			walbuf += sizeof(uint16);
+			walbuf += nitems * sizeof(ItemPointerData);
+		}
+
+		switch(a_action)
+		{
+			case GIN_SEGMENT_ADDITEMS:
+				appendStringInfo(buf, " %d (add %d items)", a_segno, nitems);
+				break;
+			case GIN_SEGMENT_DELETE:
+				appendStringInfo(buf, " %d (delete)", a_segno);
+				break;
+			case GIN_SEGMENT_INSERT:
+				appendStringInfo(buf, " %d (insert)", a_segno);
+				break;
+			case GIN_SEGMENT_REPLACE:
+				appendStringInfo(buf, " %d (replace)", a_segno);
+				break;
+			default:
+				appendStringInfo(buf, " %d unknown action %d ???", a_segno, a_action);
+				/* cannot decode unrecognized actions further */
+				return;
+		}
+	}
+}
+
 void
 gin_desc(StringInfo buf, uint8 xl_info, char *rec)
 {
@@ -70,9 +121,10 @@ gin_desc(StringInfo buf, uint8 xl_info, char *rec)
 					ginxlogRecompressDataLeaf *insertData =
 						(ginxlogRecompressDataLeaf *) payload;
 
-					appendStringInfo(buf, " unmodified: %u length: %u (compressed)",
-									 insertData->unmodifiedsize,
-									 insertData->length);
+					if (xl_info & XLR_BKP_BLOCK(0))
+						appendStringInfo(buf, " (full page image)");
+					else
+						desc_recompress_leaf(buf, insertData);
 				}
 				else
 				{
@@ -105,9 +157,10 @@ gin_desc(StringInfo buf, uint8 xl_info, char *rec)
 				ginxlogVacuumDataLeafPage *xlrec = (ginxlogVacuumDataLeafPage *) rec;
 				appendStringInfoString(buf, "Vacuum data leaf page, ");
 				desc_node(buf, xlrec->node, xlrec->blkno);
-				appendStringInfo(buf, " unmodified: %u length: %u",
-							 xlrec->data.unmodifiedsize,
-							 xlrec->data.length);
+				if (xl_info & XLR_BKP_BLOCK(0))
+					appendStringInfo(buf, " (full page image)");
+				else
+					desc_recompress_leaf(buf, &xlrec->data);
 			}
 			break;
 		case XLOG_GIN_DELETE_PAGE: