Re-think predicate locking on GIN indexes.

The principle behind the locking was not very well thought-out, and not
documented. Add a section in the README to explain how it's supposed to
work, and change the code so that it actually works that way.

This fixes two bugs:

1. If fast update was turned on concurrently, subsequent inserts to the
   pending list would not conflict with predicate locks that were acquired
   earlier, on entry pages. The included 'predicate-gin-fastupdate' test
   demonstrates that. To fix, make all scans acquire a predicate lock on
   the metapage. That lock represents a scan of the pending list, whether
   or not there is a pending list at the moment. Forget about the
   optimization to skip locking/checking for locks, when fastupdate=off.
2. If a scan finds no match, it still needs to lock the entry page. The
   point of predicate locks is to lock the gabs between values, whether
   or not there is a match. The included 'predicate-gin-nomatch' test
   tests that case.

In addition to those two bug fixes, this removes some unnecessary locking,
following the principle laid out in the README. Because all items in
a posting tree have the same key value, a lock on the posting tree root is
enough to cover all the items. (With a very large posting tree, it would
possibly be better to lock the posting tree leaf pages instead, so that a
"skip scan" with a query like "A & B", you could avoid unnecessary conflict
if a new tuple is inserted with A but !B. But let's keep this simple.)

Also, some spelling  fixes.

Author: Heikki Linnakangas with some editorization by me
Review: Andrey Borodin, Alexander Korotkov
Discussion: https://www.postgresql.org/message-id/0b3ad2c2-2692-62a9-3a04-5724f2af9114@iki.fi

1 files changed, 0 insertions, 3 deletions

diff --git a/src/include/access/gin_private.h b/src/include/access/gin_private.h
index c013d60371a..f0baac65869 100644
--- a/src/include/access/gin_private.h
+++ b/src/include/access/gin_private.h
@@ -103,8 +103,6 @@ extern Datum *ginExtractEntries(GinState *ginstate, OffsetNumber attnum,
 extern OffsetNumber gintuple_get_attrnum(GinState *ginstate, IndexTuple tuple);
 extern Datum gintuple_get_key(GinState *ginstate, IndexTuple tuple,
 				 GinNullCategory *category);
-extern void GinCheckForSerializableConflictIn(Relation relation,
-								  HeapTuple tuple, Buffer buffer);
 
 /* gininsert.c */
 extern IndexBuildResult *ginbuild(Relation heap, Relation index,
@@ -227,7 +225,6 @@ extern void ginInsertItemPointers(Relation index, BlockNumber rootBlkno,
 					  GinStatsData *buildStats);
 extern GinBtreeStack *ginScanBeginPostingTree(GinBtree btree, Relation index, BlockNumber rootBlkno, Snapshot snapshot);
 extern void ginDataFillRoot(GinBtree btree, Page root, BlockNumber lblkno, Page lpage, BlockNumber rblkno, Page rpage);
-extern void ginPrepareDataScan(GinBtree btree, Relation index, BlockNumber rootBlkno);
 
 /*
  * This is declared in ginvacuum.c, but is passed between ginVacuumItemPointers