Re: benchmarking the query planner

Simon Riggs <simon@2ndQuadrant.com> writes:

> On Thu, 2008-12-11 at 13:09 -0500, Tom Lane wrote:
>
>> On the whole I think we have some evidence here to say that upping the
>> default value of default_stats_target to 100 wouldn't be out of line,
>> but 1000 definitely is.  Comments?
>
> Sounds good to me.
>
> I would like it even more if there was a data type specific default.
> Currently we have a special case for boolean, but that's it. TPC allows
> for different defaults for different types (but that's not my only
> reason). That would allow us to set it lower for long text strings and
> floats and potentially higher for things like smallint, which is less
> likely as a join target. 

In conjunction with a toast rethink it would be interesting to engineer things
so that we keep one page's worth of data after toasting for each of the
arrays. That would at least remove a lot of the dangers of larger arrays.

> Would be great if we could set the default_stats_target for all columns
> in a table with a single ALTER TABLE statement, rather than setting it
> for every column individually.

Hm, just because one column has a skewed data distribution doesn't tell you
much about what the other columns' data distributions are. On the other hand I
could see an argument that a given table might be consistently used in only
large batch queries or quick OLTP queries.

> And I would like it even more if the sample size increased according to
> table size, since that makes ndistinct values fairly random for large
> tables.

Unfortunately _any_ ndistinct estimate based on a sample of the table is going
to be pretty random.

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