Re: bad query plans for ~ "^string" (and like "string%") (8.3.6)

From: Marinos Yannikos
Subject: Re: bad query plans for ~ "^string" (and like "string%") (8.3.6)
Date: ,
Msg-id: 49DCABC2.6030406@geizhals.at
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In response to: bad query plans for ~ "^string" (and like "string%") (8.3.6)  (Marinos Yannikos)
Responses: Re: bad query plans for ~ "^string" (and like "string%") (8.3.6)  (Tom Lane)
List: pgsql-performance


Marinos Yannikos wrote:
> (what exactly does ANALYZE look at for text columns? in our case, about
> 7% of the rows match the index condition, so it seems that left-anchored
> regexp/like matches are not evaluated using the gathered
> most-common-value list at all)

oops, I think I gave myself the answer there. Of course the
most-common-value list will not help if all the values that match the
"bad" index condition exist only once, but have a common prefix...

Perhaps Postgres could sample the first few characters separately for
such queries, but it's probably not worth it.

Regards,
  Marinos


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