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Given a query with enough joins, it was possible that the query planner, after multiplying the row estimates with the join selectivity that the estimated number of rows would exceed the limits of the double data type and become infinite. To give an indication on how extreme a case is required to hit this, the particular example case reported required 379 joins to a table without any statistics, which resulted in the 1.0/DEFAULT_NUM_DISTINCT being used for the join selectivity. This eventually caused the row estimates to go infinite and resulted in an assert failure in initial_cost_mergejoin() where the infinite row estimated was multiplied by an outerstartsel of 0.0 resulting in NaN. The failing assert verified that NaN <= Inf, which is false. To get around this we use clamp_row_est() to cap row estimates at a maximum of 1e100. This value is thought to be low enough that costs derived from it would remain within the bounds of what the double type can represent. Aside from fixing the failing Assert, this also has the added benefit of making it so add_path() will still receive proper numerical values as costs which will allow it to make more sane choices when determining the cheaper path in extreme cases such as the one described above. Additionally, we also get rid of the isnan() checks in the join costing functions. The actual case which originally triggered those checks to be added in the first place never made it to the mailing lists. It seems likely that the new code being added to clamp_row_est() will result in those becoming checks redundant, so just remove them. The fairly harmless assert failure problem does also exist in the backbranches, however, a more minimalistic fix will be applied there. Reported-by: Onder Kalaci Reviewed-by: Tom Lane Discussion: https://postgr.es/m/DM6PR21MB1211FF360183BCA901B27F04D80B0@DM6PR21MB1211.namprd21.prod.outlook.com |
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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/.