Re: Optimizing query?

Le mercredi 30 janvier 2013 à 11:08 +0000, wolfgang(at)noten5(dot)maas-noten(dot)de
a écrit :
> Hi,
>
> I am trying to match items from 2 tables based on a common string.
> One is a big table which has one column with entries like XY123, ABC44, etc
> The table has an index on that column.
> The second table is, typically, much smaller
>
> select .... from tab1, tab2 where tab1.code = tab2.code;
>
> This works fine and fast.
> Now, as a variant, I have some entries like XY423A, XY423B, GF55A, GF55D in the
> big table and want them to match XY423, GF55 in the second table
>
> Variants I have tried
>
> select .... from tab1, tab2 where tab1.code ~ (tab2.code||'($|[A-Z])');
> select .... from tab1, tab2 where tab1.code ~ ('^'||tab2.code||'($|[A-Z])');
>

Have you tried the substring function?

select .... from tab1, tab2 where substring(tab1.code from 1 for 5) =
tab2.code

> both take an enormous time. In the better case that I can subset (e.g. all candidates in table 2
> share initial "AX") I get back to manageable times by adding
> and tab1.code ~ '^AX'
> into the recipe. Actual runtime with about a million entries in tab1 and 800 entries in tab2
> is about 40 seconds.
>
> Regards
> Wolfgang Hamann
>
>
>
>
>
>

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