Re: How embarrassing: optimization of a one-shot query doesn't work

Tom,

I believe this is pretty much a show stopper for anyone using jdbc to
upgrade to 8.3.x.

Any word on 8.3.2 ?

Dave
On 31-Mar-08, at 7:26 PM, Tom Lane wrote:

> While testing the changes I was making to Pavel's EXECUTE USING patch
> to ensure that parameter values were being provided to the planner,
> it became painfully obvious that the planner wasn't actually *doing*
> anything with them. For example
>
> execute 'select count(*) from foo where x like $1' into c using $1;
>
> wouldn't generate an indexscan when $1 was of the form 'prefix%'.
>
> Some investigation showed that the planner is using the passed values
> for estimation purposes, but not for any purposes where the value
> *must*
> be correct (not only this LIKE-optimization, but constraint exclusion,
> for instance). The reason is that the parameter values are made
> available to estimate_expression_value but not to
> eval_const_expressions.
> This is a thinko in a cleanup patch I made early in 8.3 development:
> http://archives.postgresql.org/pgsql-committers/2007-02/msg00352.php
> I said to myself "eval_const_expressions doesn't need any context,
> because a constant expression's value must be independent of context,
> so I can avoid changing its API". Silly me.
>
> The implication of this is that 8.3 is significantly worse than 8.2
> in optimizing unnamed statements in the extended-Query protocol;
> a feature that JDBC, at least, relies on.
>
> The fix is simple: add PlannerInfo to eval_const_expressions's
> parameter list, as was done for estimate_expression_value. I am
> slightly hesitant to do this in a stable branch, since it would break
> any third-party code that might be calling that function. I doubt
> there
> is currently any production-grade code doing so, but if anyone out
> there
> is actively using those planner hooks we put into 8.3, it's
> conceivable
> this would affect them.
>
> Still, the performance regression here is bad enough that I think
> there
> is little choice. Comments/objections?
>
> regards, tom lane
>
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