Tom Lane wrote:
> Alvaro Herrera <alvherre@commandprompt.com> writes:
> > I'm chasing an apparent index corruption problem, and I came across
> > something I can't quite explain in pg_filedump. Say I dump a non-leaf
> > btree index page:
>
> I think this is actually OK. Remember that in a non-rightmost page,
> item 1 is the high key not a data entry. On the other hand, in a
> non-leaf page, we don't bother to store the key for the first downlink
> entry, since the associated key is really "minus infinity". Cf
> nbtree/README:
>
> On a non-leaf page, the data items are down-links to child pages with
> bounding keys. The key in each data item is the *lower* bound for
> keys on that child page, so logically the key is to the left of that
> downlink. The high key (if present) is the upper bound for the last
> downlink. The first data item on each such page has no lower bound
> --- or lower bound of minus infinity, if you prefer. The comparison
> routines must treat it accordingly. The actual key stored in the
> item is irrelevant, and need not be stored at all. This arrangement
> corresponds to the fact that an L&Y non-leaf page has one more pointer
> than key.
Ahh, I had forgotten that bit completely. Thanks.
> So item 2 doesn't have a key in it. The other two items have null
> keys, which means they need a null bitmap.
Correct.
> I don't however understand
> why there seems to be data as well as a null bitmap in there --- is
> this perhaps a two-column index?
Eh, yeah, it's a two column index, so it's OK.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
