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Re-think predicate locking on GIN indexes.

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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
This commit is contained in:
Teodor Sigaev
2018-05-04 11:27:50 +03:00
parent 7d8679975f
commit 0bef1c0678
18 changed files with 251 additions and 117 deletions
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7
src/backend/access/gin/gindatapage.c
View File

@@ -1812,8 +1812,8 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
blkno = BufferGetBlockNumber(buffer);
/*
* Copy a predicate lock from entry tree leaf (containing posting list) to
* posting tree.
* Copy any predicate locks from the entry tree leaf (containing posting
* list) to the posting tree.
*/
PredicateLockPageSplit(index, BufferGetBlockNumber(entrybuffer), blkno);
@@ -1864,7 +1864,7 @@ createPostingTree(Relation index, ItemPointerData *items, uint32 nitems,
return blkno;
}
void
static void
ginPrepareDataScan(GinBtree btree, Relation index, BlockNumber rootBlkno)
{
memset(btree, 0, sizeof(GinBtreeData));
@@ -1911,7 +1911,6 @@ ginInsertItemPointers(Relation index, BlockNumber rootBlkno,
btree.itemptr = insertdata.items[insertdata.curitem];
stack = ginFindLeafPage(&btree, false, NULL);
GinCheckForSerializableConflictIn(btree.index, NULL, stack->buffer);
ginInsertValue(&btree, stack, &insertdata, buildStats);
}
}