# Investigate if request for update statistics can be skipped for certain kind of indexes, one instance may be unique indexes based on one column.

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#### Details

• Improvement
• Status: Closed
• Major
• Resolution: Fixed
• 10.5.1.1
• None
• Performance

#### Description

DERBY-269 provided a manual way to update the statisitcs. There was some discussion in that jira entry for possibly optimizing the cases where there is no need to update the statistics. I will enter the related comments from that jira entry here for reference.

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Knut Anders Hatlen - 18/Jul/08 12:39 AM
If I have understood correctly, unique indexes always have up to date cardinality statistics because cardinality == row count. If that's the case, one possible optimization is to skip the unique indexes when SYSCS_UPDATE_STATISTICS is called.
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Mike Matrigali - 18/Jul/08 09:48 AM
is the cardinality of a unique index 1 or is it row count?

It is also more complicated than just skipping unique indexes, it depends on the number of columns in the index because
in a multi-column index, multiple cardinalities are calculated. So for instance on an index on columns A,B,C there are
actually 3 cardinalities calculated:
A
A,B
A,B,C

I agree that the calculation of cardinality of A,B,C could/should be short circuited for a unique index.
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Knut Anders Hatlen - 18/Jul/08 03:25 PM
Mike,
It looks to me as if the cardinality is the number of unique values, so I think the cardinality of a unique index is equal to its row count (for the full key, that is). You're right that we can't short circuit it if we have a multi-column index. I don't know if it's worth the extra complexity to short circuit the A,B,C case, since we'd have to scan the entire index anyway. For a single-column unique index it sounds like a good idea, though.
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#### Attachments

1. derby-3790-2a-minor_test_improvements.diff
7 kB
Kristian Waagan
2. derby-3790-1c-skip_stats_scui.diff
55 kB
Kristian Waagan
3. derby-3790-1b-skip_stats_scui.diff
55 kB
Kristian Waagan
4. derby-3790-1a-skip_stats_scui.diff
9 kB
Kristian Waagan

#### People

Kristian Waagan
Mamta A. Satoor