From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Greg Stark <gsstark(at)mit(dot)edu> |
Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, bizgres-general(at)pgfoundry(dot)org, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: A Guide to Constraint Exclusion (Partitioning) |
Date: | 2005-07-23 17:32:01 |
Message-ID: | 25487.1122139921@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Greg Stark <gsstark(at)mit(dot)edu> writes:
> Note also that the index is only useful if the index is *being used*. And
> index scans are much slower than sequential scans.
You miss my point entirely: an indexscan that hasn't got to retrieve any
rows (because it has a constraint that points off the end of the index
range) is extremely fast, and the planner will reliably detect that and
use the index scan over a seqscan (assuming it has statistics showing
the range of indexed values). And this decision is made separately for
each child table, so the fact that a seqscan might be the best bet for
the target partition doesn't stop the planner from using the indexscan
in other partitions.
However, Simon made a fair argument that there are useful cases where
you don't need an index on a partitioning key, so my objection is
answered.
regards, tom lane
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