Hi, Alena! On Mon, Oct 28, 2024 at 6:55 PM Alena Rybakina <a.rybak...@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_quals function. 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 and that there is no > underestimation, which can lead to the selection of a partial index in the > plan. See comment for the add_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_by function, > 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 are logically implied by, * the index > quals; so we also try a predicate_implied_by() check to see * if we can > discard quals that 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 and did not notice any difference in > regression tests. As I understood, our transformation is completely > equivalent, so loosy should be true. But I don't think they are needed since > our expressions are equivalent. I thought for a long time about an 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
