Fix "single value strategy" index deletion issue.

It is not appropriate for deduplication to apply single value strategy
when triggered by a bottom-up index deletion pass.  This wastes cycles
because later bottom-up deletion passes will overinterpret older
duplicate tuples that deduplication actually just skipped over "by
design".  It also makes bottom-up deletion much less effective for low
cardinality indexes that happen to cross a meaningless "index has single
key value per leaf page" threshold.

To fix, slightly narrow the conditions under which deduplication's
single value strategy is considered.  We already avoided the strategy
for a unique index, since our high level goal must just be to buy time
for VACUUM to run (not to buy space).  We'll now also avoid it when we
just had a bottom-up pass that reported failure.  The two cases share
the same high level goal, and already overlapped significantly, so this
approach is quite natural.

Oversight in commit d168b666, which added bottom-up index deletion.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznaOvM+Gyj-JQ0X=JxoMDxctDTYjiEuETdAGbF5EUc3MA@mail.gmail.com
Backpatch: 14-, where bottom-up deletion was introduced.

src/backend/access/nbtree/nbtdedup.c

@@ -34,14 +34,17 @@ static bool _bt_posting_valid(IndexTuple posting);
  *
  * The general approach taken here is to perform as much deduplication as
  * possible to free as much space as possible.  Note, however, that "single
- * value" strategy is sometimes used for !checkingunique callers, in which
- * case deduplication will leave a few tuples untouched at the end of the
- * page.  The general idea is to prepare the page for an anticipated page
- * split that uses nbtsplitloc.c's "single value" strategy to determine a
- * split point.  (There is no reason to deduplicate items that will end up on
- * the right half of the page after the anticipated page split; better to
- * handle those if and when the anticipated right half page gets its own
- * deduplication pass, following further inserts of duplicates.)
+ * value" strategy is used for !bottomupdedup callers when the page is full of
+ * tuples of a single value.  Deduplication passes that apply the strategy
+ * will leave behind a few untouched tuples at the end of the page, preparing
+ * the page for an anticipated page split that uses nbtsplitloc.c's own single
+ * value strategy.  Our high level goal is to delay merging the untouched
+ * tuples until after the page splits.
+ *
+ * When a call to _bt_bottomupdel_pass() just took place (and failed), our
+ * high level goal is to prevent a page split entirely by buying more time.
+ * We still hope that a page split can be avoided altogether.  That's why
+ * single value strategy is not even considered for bottomupdedup callers.
  *
  * The page will have to be split if we cannot successfully free at least
  * newitemsz (we also need space for newitem's line pointer, which isn't
@@ -52,7 +55,7 @@ static bool _bt_posting_valid(IndexTuple posting);
  */
 void
 _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel, IndexTuple newitem,
-			   Size newitemsz, bool checkingunique)
+			   Size newitemsz, bool bottomupdedup)
 {
 	OffsetNumber offnum,
 				minoff,
@@ -97,8 +100,11 @@ _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel, IndexTuple newitem,
 	minoff = P_FIRSTDATAKEY(opaque);
 	maxoff = PageGetMaxOffsetNumber(page);
 
-	/* Determine if "single value" strategy should be used */
-	if (!checkingunique)
+	/*
+	 * Consider applying "single value" strategy, though only if the page
+	 * seems likely to be split in the near future
+	 */
+	if (!bottomupdedup)
 		singlevalstrat = _bt_do_singleval(rel, page, state, minoff, newitem);
 
 	/*
@@ -764,14 +770,6 @@ _bt_bottomupdel_finish_pending(Page page, BTDedupState state,
  * the first pass) won't spend many cycles on the large posting list tuples
  * left by previous passes.  Each pass will find a large contiguous group of
  * smaller duplicate tuples to merge together at the end of the page.
- *
- * Note: We deliberately don't bother checking if the high key is a distinct
- * value (prior to the TID tiebreaker column) before proceeding, unlike
- * nbtsplitloc.c.  Its single value strategy only gets applied on the
- * rightmost page of duplicates of the same value (other leaf pages full of
- * duplicates will get a simple 50:50 page split instead of splitting towards
- * the end of the page).  There is little point in making the same distinction
- * here.
  */
 static bool
 _bt_do_singleval(Relation rel, Page page, BTDedupState state,

src/backend/access/nbtree/nbtinsert.c

@@ -2748,7 +2748,7 @@ _bt_delete_or_dedup_one_page(Relation rel, Relation heapRel,
 	/* Perform deduplication pass (when enabled and index-is-allequalimage) */
 	if (BTGetDeduplicateItems(rel) && itup_key->allequalimage)
 		_bt_dedup_pass(rel, buffer, heapRel, insertstate->itup,
-					   insertstate->itemsz, checkingunique);
+					   insertstate->itemsz, (indexUnchanged || uniquedup));
 }
 
 /*

src/include/access/nbtree.h

@@ -1155,7 +1155,7 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
  */
 extern void _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel,
 						   IndexTuple newitem, Size newitemsz,
-						   bool checkingunique);
+						   bool bottomupdedup);
 extern bool _bt_bottomupdel_pass(Relation rel, Buffer buf, Relation heapRel,
 								 Size newitemsz);
 extern void _bt_dedup_start_pending(BTDedupState state, IndexTuple base,