Enhance nbtree index tuple deletion.

Teach nbtree and heapam to cooperate in order to eagerly remove
duplicate tuples representing dead MVCC versions.  This is "bottom-up
deletion".  Each bottom-up deletion pass is triggered lazily in response
to a flood of versions on an nbtree leaf page.  This usually involves a
"logically unchanged index" hint (these are produced by the executor
mechanism added by commit 9dc718bd).

The immediate goal of bottom-up index deletion is to avoid "unnecessary"
page splits caused entirely by version duplicates.  It naturally has an
even more useful effect, though: it acts as a backstop against
accumulating an excessive number of index tuple versions for any given
_logical row_.  Bottom-up index deletion complements what we might now
call "top-down index deletion": index vacuuming performed by VACUUM.
Bottom-up index deletion responds to the immediate local needs of
queries, while leaving it up to autovacuum to perform infrequent clean
sweeps of the index.  The overall effect is to avoid certain
pathological performance issues related to "version churn" from UPDATEs.

The previous tableam interface used by index AMs to perform tuple
deletion (the table_compute_xid_horizon_for_tuples() function) has been
replaced with a new interface that supports certain new requirements.
Many (perhaps all) of the capabilities added to nbtree by this commit
could also be extended to other index AMs.  That is left as work for a
later commit.

Extend deletion of LP_DEAD-marked index tuples in nbtree by adding logic
to consider extra index tuples (that are not LP_DEAD-marked) for
deletion in passing.  This increases the number of index tuples deleted
significantly in many cases.  The LP_DEAD deletion process (which is now
called "simple deletion" to clearly distinguish it from bottom-up
deletion) won't usually need to visit any extra table blocks to check
these extra tuples.  We have to visit the same table blocks anyway to
generate a latestRemovedXid value (at least in the common case where the
index deletion operation's WAL record needs such a value).

Testing has shown that the "extra tuples" simple deletion enhancement
increases the number of index tuples deleted with almost any workload
that has LP_DEAD bits set in leaf pages.  That is, it almost never fails
to delete at least a few extra index tuples.  It helps most of all in
cases that happen to naturally have a lot of delete-safe tuples.  It's
not uncommon for an individual deletion operation to end up deleting an
order of magnitude more index tuples compared to the old naive approach
(e.g., custom instrumentation of the patch shows that this happens
fairly often when the regression tests are run).

Add a further enhancement that augments simple deletion and bottom-up
deletion in indexes that make use of deduplication: Teach nbtree's
_bt_delitems_delete() function to support granular TID deletion in
posting list tuples.  It is now possible to delete individual TIDs from
posting list tuples provided the TIDs have a tableam block number of a
table block that gets visited as part of the deletion process (visiting
the table block can be triggered directly or indirectly).  Setting the
LP_DEAD bit of a posting list tuple is still an all-or-nothing thing,
but that matters much less now that deletion only needs to start out
with the right _general_ idea about which index tuples are deletable.

Bump XLOG_PAGE_MAGIC because xl_btree_delete changed.

No bump in BTREE_VERSION, since there are no changes to the on-disk
representation of nbtree indexes.  Indexes built on PostgreSQL 12 or
PostgreSQL 13 will automatically benefit from bottom-up index deletion
(i.e. no reindexing required) following a pg_upgrade.  The enhancement
to simple deletion is available with all B-Tree indexes following a
pg_upgrade, no matter what PostgreSQL version the user upgrades from.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wzm+maE3apHB8NOtmM=p-DO65j2V5GzAWCOEEuy3JZgb2g@mail.gmail.com

src/include/access/nbtree.h

@@ -17,6 +17,7 @@
 #include "access/amapi.h"
 #include "access/itup.h"
 #include "access/sdir.h"
+#include "access/tableam.h"
 #include "access/xlogreader.h"
 #include "catalog/pg_am_d.h"
 #include "catalog/pg_index.h"
@@ -168,7 +169,7 @@ typedef struct BTMetaPageData
 /*
  * MaxTIDsPerBTreePage is an upper bound on the number of heap TIDs tuples
  * that may be stored on a btree leaf page.  It is used to size the
- * per-page temporary buffers used by index scans.
+ * per-page temporary buffers.
  *
  * Note: we don't bother considering per-tuple overheads here to keep
  * things simple (value is based on how many elements a single array of
@@ -766,8 +767,9 @@ typedef struct BTDedupStateData
 typedef BTDedupStateData *BTDedupState;
 
 /*
- * BTVacuumPostingData is state that represents how to VACUUM a posting list
- * tuple when some (though not all) of its TIDs are to be deleted.
+ * BTVacuumPostingData is state that represents how to VACUUM (or delete) a
+ * posting list tuple when some (though not all) of its TIDs are to be
+ * deleted.
  *
  * Convention is that itup field is the original posting list tuple on input,
  * and palloc()'d final tuple used to overwrite existing tuple on output.
@@ -1031,6 +1033,8 @@ 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);
+extern bool _bt_bottomupdel_pass(Relation rel, Buffer buf, Relation heapRel,
+								 Size newitemsz);
 extern void _bt_dedup_start_pending(BTDedupState state, IndexTuple base,
 									OffsetNumber baseoff);
 extern bool _bt_dedup_save_htid(BTDedupState state, IndexTuple itup);
@@ -1045,7 +1049,8 @@ extern IndexTuple _bt_swap_posting(IndexTuple newitem, IndexTuple oposting,
  * prototypes for functions in nbtinsert.c
  */
 extern bool _bt_doinsert(Relation rel, IndexTuple itup,
-						 IndexUniqueCheck checkUnique, Relation heapRel);
+						 IndexUniqueCheck checkUnique, bool indexUnchanged,
+						 Relation heapRel);
 extern void _bt_finish_split(Relation rel, Buffer lbuf, BTStack stack);
 extern Buffer _bt_getstackbuf(Relation rel, BTStack stack, BlockNumber child);
 
@@ -1083,9 +1088,9 @@ extern bool _bt_page_recyclable(Page page);
 extern void _bt_delitems_vacuum(Relation rel, Buffer buf,
 								OffsetNumber *deletable, int ndeletable,
 								BTVacuumPosting *updatable, int nupdatable);
-extern void _bt_delitems_delete(Relation rel, Buffer buf,
-								OffsetNumber *deletable, int ndeletable,
-								Relation heapRel);
+extern void _bt_delitems_delete_check(Relation rel, Buffer buf,
+									  Relation heapRel,
+									  TM_IndexDeleteOp *delstate);
 extern uint32 _bt_pagedel(Relation rel, Buffer leafbuf,
 						  TransactionId *oldestBtpoXact);