Tom Lane wrote:
Heikki Linnakangas <[EMAIL PROTECTED]> writes:
Martijn van Oosterhout wrote:
IIRC indexes can already ask to have the system recheck conditions on
returned tuples. For example GiST can return more tuples than actually
match. That's what the amopreqcheck column is for in pg_amop.


Right, except that flag is per operator in operator class, and what I'm proposing is that the index could pass a flag per tuple in the scan.

The reason for attaching the flag to operators is so that the system
(particularly the planner) can tell *which* conditions need to be
rechecked, and can prepare the necessary expression infrastructure.
I dislike the idea of having to be prepared to do that every time
for every indexscan.

I don't see any big down-side in preparing for that. We'd need to always store the original index quals in the executor node, like we do now with recheck-flagged operators, but that doesn't seem too bad to me.

I suppose we would want to keep the existing per-operator recheck-flag and quals as it is, and add another field like indexqualorig to be used to recheck tuples amgetnext flags as candidates.

The notion of having to be prepared to sort
(according to what?) is even worse.

That we wouldn't need for clustered indexes, if we change the current design a bit. Either:
* store a sorted list of offsetnumbers for each group, instead of a bitmap,
* or store a bitmap like now, but require that heap tuples in a grouped index tuple are in cluster order within the heap page.

The first option eats away some of the space savings, the second option makes clustered indexes to become declustered quicker if there's out-of-order updates or inserts.

Choosing either option would also reduce the CPU overhead of index scans, because we could use binary search within a grouped index tuple.

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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