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Commit 5e0928005 changed the planner so that, instead of blindly using DEFAULT_COLLATION_OID when invoking operators for selectivity estimation, it would use the collation of the column whose statistics we're considering. This was recognized as still being not quite the right thing, but it seemed like a good incremental improvement. However, shortly thereafter we introduced nondeterministic collations, and that creates cases where operators can fail if they're passed the wrong collation. We don't want planning to fail in cases where the query itself would work, so this means that we *must* use the query's collation when invoking operators for estimation purposes. The only real problem this creates is in ineq_histogram_selectivity, where the binary search might produce a garbage answer if we perform comparisons using a different collation than the column's histogram is ordered with. However, when the query's collation is significantly different from the column's default collation, the estimate we previously generated would be pretty irrelevant anyway; so it's not clear that this will result in noticeably worse estimates in practice. (A follow-on patch will improve this situation in HEAD, but it seems too invasive for back-patch.) The patch requires changing the signatures of mcv_selectivity and allied functions, which are exported and very possibly are used by extensions. In HEAD, I just did that, but an API/ABI break of this sort isn't acceptable in stable branches. Therefore, in v12 the patch introduces "mcv_selectivity_ext" and so on, with signatures matching HEAD, and makes the old functions into wrappers that assume DEFAULT_COLLATION_OID should be used. That does not match the prior behavior, but it should avoid risk of failure in most cases. (In practice, I think most extension datatypes aren't collation-aware, so the change probably doesn't matter to them.) Per report from James Lucas. Back-patch to v12 where the problem was introduced. Discussion: https://postgr.es/m/CAAFmbbOvfi=wMM=3qRsPunBSLb8BFREno2oOzSBS=mzfLPKABw@mail.gmail.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.