Re: Failure while inserting parent tuple to B-tree is not fun

On 2013-10-22 19:55:09 +0300, Heikki Linnakangas wrote:
> Splitting a B-tree page is a two-stage process: First, the page is split,
> and then a downlink for the new right page is inserted into the parent
> (which might recurse to split the parent page, too). What happens if
> inserting the downlink fails for some reason? I tried that out, and it turns
> out that it's not nice.
>
> I used this to cause a failure:
>
> >--- a/src/backend/access/nbtree/nbtinsert.c
> >+++ b/src/backend/access/nbtree/nbtinsert.c
> >@@ -1669,6 +1669,8 @@ _bt_insert_parent(Relation rel,
> > _bt_relbuf(rel, pbuf);
> > }
> >
> >+ elog(ERROR, "fail!");
> >+
> > /* get high key from left page == lowest key on new right page */
> > ritem = (IndexTuple) PageGetItem(page,
> > PageGetItemId(page, P_HIKEY));
>
> postgres=# create table foo (i int4 primary key);
> CREATE TABLE
> postgres=# insert into foo select generate_series(1, 10000);
> ERROR: fail!
>
> That's not surprising. But when I removed that elog again and restarted the
> server, I still can't insert. The index is permanently broken:
>
> postgres=# insert into foo select generate_series(1, 10000);
> ERROR: failed to re-find parent key in index "foo_pkey" for split pages 4/5
>
> In real life, you would get a failure like this e.g if you run out of memory
> or disk space while inserting the downlink to the parent. Although rare in
> practice, it's no fun if it happens.

Why doesn't the incomplete split mechanism prevent this? Because we do
not delay checkpoints on the primary and a checkpoint happened just
befor your elog(ERROR) above?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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