733f20df53
genericcostestimate() estimates the number of index leaf pages to be visited as a pro-rata fraction of the total number of leaf pages. Or at least that was the intention. What it actually used in the calculation was the total number of index pages, so that non-leaf pages were also counted. In a decent-sized index the error is probably small, since we expect upper page fanout to be high. But in a small index that's not true; in the worst case with one data-bearing page plus a metapage, we had 100% relative error. This led to surprising planning choices such as not using a small partial index. To fix, ask genericcostestimate's caller to supply an estimate of the number of non-leaf pages, and subtract that. For the built-in index AMs, it seems sufficient to count the index metapage (if the AM uses one) as non-leaf. Per the above argument, counting upper index pages shouldn't change the estimate much, and in most cases we don't have any easy way of estimating the number of upper pages. This might be an area for further research in future. Any external genericcostestimate callers that do not set the new field GenericCosts.numNonLeafPages will see the same behavior as before, assuming they followed the advice to zero out that whole struct. Unsurprisingly, this change affects a number of plans seen in the core regression tests. I hacked up the existing tests to keep the tests' plans the same, since in each case it appeared that the test's intent was to test exactly that plan. Also add one new test case demonstrating that a better index choice is now made. Author: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Henson Choi <assam258@gmail.com> Discussion: https://postgr.es/m/870521.1745860752@sss.pgh.pa.us |
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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.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.