From: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
---|---|
To: | Alban Hertroys <dalroi(at)solfertje(dot)student(dot)utwente(dot)nl> |
Cc: | Mike Christensen <mike(at)kitchenpc(dot)com>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Difference between array column type and separate table |
Date: | 2009-05-03 16:19:43 |
Message-ID: | 49FDC41F.6030202@cheapcomplexdevices.com |
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Lists: | pgsql-general |
Alban Hertroys wrote:
> On May 2, 2009, at 9:33 AM, Mike Christensen wrote:
>
>> ...
>> create table Threads ( ... Tags int2[], ...);
>>
>> To me this seems cleaner, but I'm wondering about performance. If I
>> had millions of threads, is a JOIN going to be faster? ...
>
> ...I don't think array values are indexable either. ...
Of course they are indexable if you construct the right index
http://www.postgresql.org/docs/current/static/intarray.html
For certain queries, the array will probably be *much* faster
than the extra tables. For example a query like this:
SELECT * FROM Threads WHERE Threads.tags @@ '1&(2|3)'::query_int;
that does a single indexscan will be far far faster using an
extra table, where the query would look something like:
select * from Threads T where
id in (select threadid from threadtags where id = 1)
and
id in (select threadid from threadtags where id = 2 or id = 3);
requiring 3 indexscans on a table with many more rows, and
some potentially rather expensive joins.
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