Re: Use extended statistics to estimate (Var op Var) clauses - Mailing list pgsql-hackers
From | Mark Dilger |
---|---|
Subject | Re: Use extended statistics to estimate (Var op Var) clauses |
Date | |
Msg-id | CCD1B8BF-3376-454C-A2F3-9AB57A401A57@enterprisedb.com Whole thread Raw |
In response to | Re: Use extended statistics to estimate (Var op Var) clauses (Tomas Vondra <tomas.vondra@enterprisedb.com>) |
Responses |
Re: Use extended statistics to estimate (Var op Var) clauses
(Zhihong Yu <zyu@yugabyte.com>)
Re: Use extended statistics to estimate (Var op Var) clauses (Tomas Vondra <tomas.vondra@enterprisedb.com>) |
List | pgsql-hackers |
> On Aug 28, 2021, at 6:52 AM, Tomas Vondra <tomas.vondra@enterprisedb.com> wrote: > > Part 0003 fixes handling of those clauses so that we don't treat them as simple, but it does that by tweaking statext_is_compatible_clause(),as suggested by Dean. Function examine_opclause_args() doesn't set issimple to anything in the IsA(rightop, Const) case, but assigns *issimplep= issimple at the bottom. The compiler is not complaining about using a possibly uninitialized variable, but ifI change the "return true" on the very next line to "return issimple", the compiler complains quite loudly. Some functions define bool *issimple, others bool *issimplep and bool issimple. You might want to standardize the naming. It's difficult to know what "simple" means in extended_stats.c. There is no file-global comment explaining the concept,and functions like compare_scalars_simple don't have correlates named compare_scalars_complex or such, so the readercannot infer by comparison what the difference might be between a "simple" case and some non-"simple" case. The functions'issimple (or issimplep) argument are undocumented. There is a comment: /* * statext_mcv_clauselist_selectivity * Estimate clauses using the best multi-column statistics. .... * * - simple selectivity: Computed without extended statistics, i.e. as if the * columns/clauses were independent. * .... */ but it takes a while to find if you search for "issimple". In both scalarineqsel_wrapper() and eqsel_internal(), the call to matching_restriction_variables() should usually returnfalse, since comparing a variable to itself is an unusual case. The next call is to get_restriction_variable(), whichrepeats the work of examining the left and right variables. So in almost all cases, after throwing away the resultsof: examine_variable(root, left, varRelid, &ldata); examine_variable(root, right, varRelid, &rdata); performed in matching_restriction_variables(), we'll do exactly the same work again (with one variable named differently)in get_restriction_variable(): examine_variable(root, left, varRelid, vardata); examine_variable(root, right, varRelid, &rdata); That'd be fine if example_variable() were a cheap function, but it appears not to be. Do you think you could save the resultsrather than recomputing them? It's a little messy, since these are the only two functions out of about ten whichfollow this pattern, so you'd have to pass NULLs into get_restriction_variable() from the other eight callers, but itstill looks like that would be a win. — Mark Dilger EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
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