v13-0002-Stop-deduplicating-when-a-page-split-is-avoided.patch

From a7d4cafc92358e6095a48c0b42ccbe06b7b8bd5f Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <pg@bowt.ie>
Date: Thu, 12 Sep 2019 16:19:54 -0700
Subject: [PATCH v13 2/3] Stop deduplicating when a page split is avoided.

Currently this is a big loss for performance, especially with WAL-logged
relations, though it barely affects total space utilization compared to
recent versions of the patch.  With incremental rewriting of the page
and incremental WAL logging, this could actually be a win for
performance.

In any case it seems like a good thing for deduplication to be able to
operate in a "goal-orientated" way.  The exact details will need to be
validated by extensive benchmarking.
---
 src/backend/access/nbtree/nbtinsert.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index 52651fcbe4..f3b945edf9 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -2675,6 +2675,21 @@ _bt_dedup_one_page(Relation rel, Buffer buffer, Relation heapRel,
 			pagesaving += _bt_dedup_insert(newpage, dedupState);
 		}
 
+		/*
+		 * When we have deduplicated enough to avoid page split, don't bother
+		 * deduplicating any more items.
+		 *
+		 * FIXME: If rewriting the page and doing the WAL logging were
+		 * incremental, we could actually break out of the loop and save real
+		 * work.  As things stand this is a loss for performance, but it
+		 * barely affects space utilization. (The number of blocks are the
+		 * same as before, except for rounding effects.  The minimum number of
+		 * items on each page for each index "increases" when this is enabled,
+		 * however.)
+		 */
+		if (pagesaving >= newitemsz)
+			deduplicate = false;
+
 		pfree(dedupState->itupprev);
 		dedupState->itupprev = CopyIndexTuple(itup);
 	}
-- 
2.17.1