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As with the previous patch, large numbers of null rows could skew this calculation unfavorably, causing us to discard values that have a legitimate claim to be MCVs, since our definition of MCV is that it's most common among the non-null population of the column. Hence, make the numerator of avgcount be the number of non-null sample values not the number of sample rows; likewise for maxmincount in the compute_scalar_stats variant. Also, make the denominator be the number of distinct values actually observed in the sample, rather than reversing it back out of the computed stadistinct. This avoids depending on the accuracy of the Haas-Stokes approximation, and really it's what we want anyway; the threshold should depend only on what we see in the sample, not on what we extrapolate about the contents of the whole column. Alex Shulgin, reviewed by Tomas Vondra and myself |
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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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.