Re: POC, WIP: OR-clause support for indexes - Mailing list pgsql-hackers

Hi, Alena!

On Mon, Oct 28, 2024 at 6:55 PM Alena Rybakina
<a.rybakina@postgrespro.ru> wrote:
> I may be wrong, but the original idea was to double-check the result with the original expression.
>
> But I'm willing to agree with you. I think we should add transformed rinfo variable through
add_predicate_to_index_qualsfunction. I attached the diff file to the letter. 
>
> diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
> index 3da7ea8ed57..c68ac7008e6 100644
> --- a/src/backend/optimizer/path/indxpath.c
> +++ b/src/backend/optimizer/path/indxpath.c
> @@ -3463,10 +3463,11 @@ match_orclause_to_indexcol(PlannerInfo *root,
>       * rinfo in iclause->rinfo to detect duplicates and recheck the original
>       * clause.
>       */
> +    RestrictInfo *rinfo_new = make_simple_restrictinfo(root,
> +                                                &saopexpr->xpr);
>      iclause = makeNode(IndexClause);
> -    iclause->rinfo = rinfo;
> -    iclause->indexquals = list_make1(make_simple_restrictinfo(root,
> -                                                              &saopexpr->xpr));
> +    iclause->rinfo = rinfo_new;
> +    iclause->indexquals = add_predicate_to_index_quals(index, list_make1(rinfo_new));
>      iclause->lossy = false;
>      iclause->indexcol = indexcol;
>      iclause->indexcols = NIL;

As I stated in [1], I don't think we should pass transformed clause to
IndexClause.rinfo while comment explicitly says us to pass original
rinfo there.

> I figured out comments that you mentioned and found some addition explanation.
>
> As I understand it, this processing is related to ensuring that the selectivity of the index is assessed correctly
andthat there is no underestimation, which can lead to the selection of a partial index in the plan. See comment for
theadd_predicate_to_index_quals function: 
>
> * ANDing the index predicate with the explicitly given indexquals produces
>  * a more accurate idea of the index's selectivity.  However, we need to be
>  * careful not to insert redundant clauses, because clauselist_selectivity()
>  * is easily fooled into computing a too-low selectivity estimate.  Our
>  * approach is to add only the predicate clause(s) that cannot be proven to
>  * be implied by the given indexquals.  This successfully handles cases such
>  * as a qual "x = 42" used with a partial index "WHERE x >= 40 AND x < 50".
>  * There are many other cases where we won't detect redundancy, leading to a
>  * too-low selectivity estimate, which will bias the system in favor of using
>  * partial indexes where possible.  That is not necessarily bad though.
>  *
>  * Note that indexQuals contains RestrictInfo nodes while the indpred
>  * does not, so the output list will be mixed.  This is OK for both
>  * predicate_implied_by() and clauselist_selectivity(), but might be
>  * problematic if the result were passed to other things.
>  */
>
> In those comments that you mentioned, it was written that this problem of expression redundancy is checked using the
predicate_implied_byfunction, note that it is called there. 
>
> * In some situations (particularly with OR'd index conditions) we may * have scan_clauses that are not equal to, but
arelogically implied by, * the index quals; so we also try a predicate_implied_by() check to see * if we can discard
qualsthat way. (predicate_implied_by assumes its * first input contains only immutable functions, so we have to check *
that.)

As the first line of header comment of add_predicate_to_index_quals()
says it adds partial index predicate to the quals list.  I don't see
why should we use that in match_orclause_to_indexcol(), because this
function is only responsible to matching rinfo to particular index
column.  Matching of partial index predicate is handled elsewhere.
Also check there is get_index_clause_from_support(), which is fetch
transformed clause from a support function.  And it doesn't have to
fiddle with add_predicate_to_index_quals().

> I also figured out more information about loosy variable. First of all, I tried changing the value of the variable
anddid not notice any difference in regression tests. As I understood, our transformation is completely equivalent, so
loosyshould be true. But I don't think they are needed since our expressions are equivalent. I thought for a long time
aboutan example where this could be a mistake and didn’t come up with any of them. 

Yes, our transformation isn't lossy, thus IndexClause.lossy should be unset.

Links
1. https://www.postgresql.org/message-id/CAPpHfdvjtEWqjVcPd3-JQw8yCoppMXjK8kHnvinxBXGMZt-M_g%40mail.gmail.com

------
Regards,
Alexander Korotkov
Supabase