Deprecate nbtree's BTP_HAS_GARBAGE flag.

Streamline handling of the various strategies that we have to avoid a
page split in nbtinsert.c.  When it looks like a leaf page is about to
overflow, we now perform deleting LP_DEAD items and deduplication in one
central place.  This greatly simplifies _bt_findinsertloc().

This has an independently useful consequence: nbtree no longer relies on
the BTP_HAS_GARBAGE page level flag/hint for anything important.  We
still set and unset the flag in the same way as before, but it's no
longer treated as a gating condition when considering if we should check
for already-set LP_DEAD bits.  This happens at the point where the page
looks like it might have to be split anyway, so simply checking the
LP_DEAD bits in passing is practically free.  This avoids missing
LP_DEAD bits just because the page-level hint is unset, which is
probably reasonably common (e.g. it happens when VACUUM unsets the
page-level flag without actually removing index tuples whose LP_DEAD-bit
was set recently, after the VACUUM operation began but before it reached
the leaf page in question).

Note that this isn't a big behavioral change compared to PostgreSQL 13.
We were already checking for set LP_DEAD bits regardless of whether the
BTP_HAS_GARBAGE page level flag was set before we considered doing a
deduplication pass.  This commit only goes slightly further by doing the
same check for all indexes, even indexes where deduplication won't be
performed.

We don't completely remove the BTP_HAS_GARBAGE flag.  We still rely on
it as a gating condition with pg_upgrade'd indexes from before B-tree
version 4/PostgreSQL 12.  That makes sense because we sometimes have to
make a choice among pages full of duplicates when inserting a tuple with
pre version 4 indexes.  It probably still pays to avoid accessing the
line pointer array of a page there, since it won't yet be clear whether
we'll insert on to the page in question at all, let alone split it as a
result.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wz%3DYpc1PDdk8OVJDChGJBjT06%3DA0Mbv9HyTLCsOknGcUFg%40mail.gmail.com

1 files changed, 4 insertions, 4 deletions

diff --git a/src/include/access/nbtree.h b/src/include/access/nbtree.h
index 65d9698b899..e8fecc6026f 100644
--- a/src/include/access/nbtree.h
+++ b/src/include/access/nbtree.h
@@ -75,7 +75,7 @@ typedef BTPageOpaqueData *BTPageOpaque;
 #define BTP_META		(1 << 3)	/* meta-page */
 #define BTP_HALF_DEAD	(1 << 4)	/* empty, but still in tree */
 #define BTP_SPLIT_END	(1 << 5)	/* rightmost page of split group */
-#define BTP_HAS_GARBAGE (1 << 6)	/* page has LP_DEAD tuples */
+#define BTP_HAS_GARBAGE (1 << 6)	/* page has LP_DEAD tuples (deprecated) */
 #define BTP_INCOMPLETE_SPLIT (1 << 7)	/* right sibling's downlink is missing */
 
 /*
@@ -1027,9 +1027,9 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
 /*
  * prototypes for functions in nbtdedup.c
  */
-extern void _bt_dedup_one_page(Relation rel, Buffer buf, Relation heapRel,
-							   IndexTuple newitem, Size newitemsz,
-							   bool checkingunique);
+extern void _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel,
+						   IndexTuple newitem, Size newitemsz,
+						   bool checkingunique);
 extern void _bt_dedup_start_pending(BTDedupState state, IndexTuple base,
 									OffsetNumber baseoff);
 extern bool _bt_dedup_save_htid(BTDedupState state, IndexTuple itup);