Thread: Forcing use of specific index

Is it any way to attempt to force the planner to use some specific index
while creating the plan?  Other than eventually dropping all the other
indices (which is obiously not a solution in production setting anyway)?

I have one case where I have added 16 indices to a table, many of them
beeing partial indices.  The table itself has only 50k of rows, but are
frequently used in heavy joins.  I imagine there can be exponential order on
the number of alternative paths the planner must examinate as function of
the number of indices?

It seems to me that the planner is quite often not choosing the "best"
index, so I wonder if there is any easy way for me to check out what the
planner think about a specific index :-)

--
Tobias Brox, Beijing

A pretty awful way is to mangle the sql statement so the other field
logical statements are like so:

select * from mytable where 0+field = 100




Tobias Brox wrote:
> Is it any way to attempt to force the planner to use some specific index
> while creating the plan?  Other than eventually dropping all the other
> indices (which is obiously not a solution in production setting anyway)?
>
> I have one case where I have added 16 indices to a table, many of them
> beeing partial indices.  The table itself has only 50k of rows, but are
> frequently used in heavy joins.  I imagine there can be exponential order on
> the number of alternative paths the planner must examinate as function of
> the number of indices?
>
> It seems to me that the planner is quite often not choosing the "best"
> index, so I wonder if there is any easy way for me to check out what the
> planner think about a specific index :-)
>

HI all,
I also would like to know if there is a way to force a use of a
specific index for a specific query. I am currently using Postgresql
7.4.6

In my case I have a relatively big table (several millions of records)
that are frequently used to join with other tables (explicit join or
through view).
The table has several indices, some are single column and some are multi column.
Some queries are faster if using single colum index while other are
faster using multi column indexes.
I have play around with SET STATISTICS, but it doesn't seem to make
any differences (I tried to set it to 1000 one time, but still the
same). I did analyze and vacuum after SET STATISTICS.
Any pointer on how to do this is greatly appreciated.
Thank you in advance,


J



On 6/1/05, Tobias Brox <tobias@nordicbet.com> wrote:
> Is it any way to attempt to force the planner to use some specific index
> while creating the plan?  Other than eventually dropping all the other
> indices (which is obiously not a solution in production setting anyway)?
>
> I have one case where I have added 16 indices to a table, many of them
> beeing partial indices.  The table itself has only 50k of rows, but are
> frequently used in heavy joins.  I imagine there can be exponential order on
> the number of alternative paths the planner must examinate as function of
> the number of indices?
>
> It seems to me that the planner is quite often not choosing the "best"
> index, so I wonder if there is any easy way for me to check out what the
> planner think about a specific index :-)
>
> --
> Tobias Brox, Beijing
>
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"Tobias Brox" <tobias@nordicbet.com> writes
> Is it any way to attempt to force the planner to use some specific index
> while creating the plan?  Other than eventually dropping all the other
> indices (which is obiously not a solution in production setting anyway)?
>

I don't think currently PG supports this but "SQL hints" is planned ...

Regards,
Qingqing