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https://issues.apache.org/jira/browse/LUCENE-2829?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12973889#action_12973889
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Robert Muir commented on LUCENE-2829:
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Hoss Man, well I think if you surely know its a PK field you can definitely do
something better, starting with a custom collector that does something like
what you mentioned, with no PQ at all etc.
But in this case, though i categorized it as PK, the general problem is this:
* in lots of cases we do redundant seeks, like to get the docFreq, then to get
the DocsEnum
* in most cases the term dictionary cache helps here because the 2nd time (e.g.
getting DocsEnum) is cached.
Here's the problem with "PK" or "PK-ish" (low freq terms like what
wildcards/fuzzies/range queries hit too):
* our cache doesn't cache "negative" hits, the fact that a term *doesnt* exist
in some segment.
* For example in the PK case, if there are 15 segments we always get at most 1
cache hit and
at least 14 misses when getting the DocsEnum, so we do at least 14 wasted seeks
always.
* For other low frequency terms that don't exist in all segments (very precise
dates or what have you)
the same idea applies, just to a lesser extent: the PK is the worst.
> improve termquery "pk lookup" performance
> -----------------------------------------
>
> Key: LUCENE-2829
> URL: https://issues.apache.org/jira/browse/LUCENE-2829
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Reporter: Robert Muir
> Attachments: LUCENE-2829.patch
>
>
> For things that are like primary keys and don't exist in some segments (worst
> case is primary/unique key that only exists in 1)
> we do wasted seeks.
> While LUCENE-2694 tries to solve some of this issue with TermState, I'm
> concerned we could every backport that to 3.1 for example.
> This is a simpler solution here just to solve this one problem in
> termquery... we could just revert it in trunk when we resolve LUCENE-2694,
> but I don't think we should leave things as they are in 3.x
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