Re: [GENERAL] BIGINT indexes still with problems

[Peter Eisentraut]

Dan Ruthers wrote:
> The index is used!
> I also did a vacuum analyze, and restarted Postgres and it did not
> make any difference. I tried many other ID values (ex 783218 and
> 783220), and they seem to use the index correctly. Only that value
> doesn't.

Possibly, that is the most common value and the cost calculation yields 
that it would be more efficient to not use the index.  If you disagree, 
please show the timings generated by EXPLAIN ANALYZE.

-- 
Peter Eisentraut
http://developer.postgresql.org/~petere/


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Re: [GENERAL] BIGINT indexes still with problems

[Stephan Szabo]

On Mon, 9 Aug 2004, Dan Ruthers wrote:

> Now, if I run this query (note the int8 cast - also tried with the '' cast to 
> String, same results):
> test=> explain select * from dmaildatum where idparent=int8(783219);
> QUERY PLAN
> --
>  Seq Scan on dmaildatum  (cost=0.00..2241.71 rows=2229 width=272)
>Filter: (idparent = 783219::bigint)
> (2 rows)
>
> The index is not used. But with an identical query, only different parameter value:
> desknow=> explain select * from dmaildatum where idparent=int8(1187838);
>   QUERY PLAN
>
> 
> ---
>  Index Scan using ix_dmaildatum_idparent on dmaildatum  (cost=0.00..284.05 rows=
> 102 width=272)
>Index Cond: (idparent = 1187838::bigint)
> (2 rows)


Look at the row estimates for the two cases.  How many rows are actually
returned and how long the queries take (explain analyze will give that
information)?


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Re: [GENERAL] BIGINT indexes still with problems

[Gaetano Mendola]

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Dan Ruthers wrote:
| Now, if I run this query (note the int8 cast - also tried with the '' cast to 
String, same results):
| test=> explain select * from dmaildatum where idparent=int8(783219);
| QUERY PLAN
| --
|  Seq Scan on dmaildatum  (cost=0.00..2241.71 rows=2229 width=272)
|Filter: (idparent = 783219::bigint)
| (2 rows)
|
| The index is not used. But with an identical query, only different parameter value:
| desknow=> explain select * from dmaildatum where idparent=int8(1187838);
|   QUERY PLAN
|
| 
| ---
|  Index Scan using ix_dmaildatum_idparent on dmaildatum  (cost=0.00..284.05 rows=
| 102 width=272)
|Index Cond: (idparent = 1187838::bigint)
| (2 rows)
|
| The index is used!
| I also did a vacuum analyze, and restarted Postgres and it did not make any 
difference.
| I tried many other ID values (ex 783218 and 783220), and they seem to use the index 
correctly. Only that value doesn't.
|
| Can anyone explain why Postgres behaves differently in these two cases, or at least 
point to some hints?
Because this means that a sequential scan is better for that value.
Perform this selects:
(1) select count(*) from dmaildatum;
(2) select count(*) from dmaildatum where idparent=int8(783219);
(3) select count(*) from dmaildatum where idparent=int8(1187838);
I bet that the ratio  (2)/(1) is greater then (3)/(1).
Now show us the following results:
explain analyze select * from dmaildatum where idparent=int8(783219);
explain analyze select * from dmaildatum where idparent=int8(1187838);
and repeat it again but executing before:
set enable_seqscan = off;

Depending on the results that you get may be you need to lower the index
scan cost tuning the cpu related GUC variables.

Regards
Gaetano Mendola






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