From 587b6aa3f3cea5d30e65317c34d0cae288edaa6d Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Wed, 19 Feb 2025 23:51:48 +0100 Subject: [PATCH] Correct relation size estimate with low fillfactor Since commit 29cf61ade3, table_block_relation_estimate_size() considers fillfactor when estimating number of rows in a relation before the first ANALYZE. The formula however did not consider tuples may be larger than available space determined by fillfactor, ending with density 0. This ultimately means the relation was estimated to contain a single row. The executor however places at least one tuple per page, even with very low fillfactor values, so the density should be at least 1. Fixed by clamping the density estimate using clamp_row_est(). Reported by Heikki Linnakangas. Fix by me, with regression test inspired by example provided by Heikki. Backpatch to 17, where the issue was introduced. Reported-by: Heikki Linnakangas Backpatch-through: 17 Discussion: https://postgr.es/m/2bf9d973-7789-4937-a7ca-0af9fb49c71e@iki.fi --- src/backend/access/table/tableam.c | 3 +++ src/test/regress/expected/stats.out | 17 +++++++++++++++++ src/test/regress/sql/stats.sql | 16 ++++++++++++++++ 3 files changed, 36 insertions(+) diff --git a/src/backend/access/table/tableam.c b/src/backend/access/table/tableam.c index e57a0b7ea31..3ec11fb52c0 100644 --- a/src/backend/access/table/tableam.c +++ b/src/backend/access/table/tableam.c @@ -24,6 +24,7 @@ #include "access/syncscan.h" #include "access/tableam.h" #include "access/xact.h" +#include "optimizer/optimizer.h" #include "optimizer/plancat.h" #include "port/pg_bitutils.h" #include "storage/bufmgr.h" @@ -740,6 +741,8 @@ table_block_relation_estimate_size(Relation rel, int32 *attr_widths, tuple_width += overhead_bytes_per_tuple; /* note: integer division is intentional here */ density = (usable_bytes_per_page * fillfactor / 100) / tuple_width; + /* There's at least one row on the page, even with low fillfactor. */ + density = clamp_row_est(density); } *tuples = rint(density * (double) curpages); diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out index 6e08898b183..43b7873c62b 100644 --- a/src/test/regress/expected/stats.out +++ b/src/test/regress/expected/stats.out @@ -1646,4 +1646,21 @@ SELECT COUNT(*) FROM brin_hot_3 WHERE a = 2; DROP TABLE brin_hot_3; SET enable_seqscan = on; +-- Test that estimation of relation size works with tuples wider than the +-- relation fillfactor. We create a table with wide inline attributes and +-- low fillfactor, insert rows and then see how many rows EXPLAIN shows +-- before running analyze. We disable autovacuum so that it does not +-- interfere with the test. +CREATE TABLE table_fillfactor ( + n char(1000) +) with (fillfactor=10, autovacuum_enabled=off); +INSERT INTO table_fillfactor +SELECT 'x' FROM generate_series(1,1000); +SELECT * FROM check_estimated_rows('SELECT * FROM table_fillfactor'); + estimated | actual +-----------+-------- + 1000 | 1000 +(1 row) + +DROP TABLE table_fillfactor; -- End of Stats Test diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql index d8ac0d06f48..ed0bc1ef074 100644 --- a/src/test/regress/sql/stats.sql +++ b/src/test/regress/sql/stats.sql @@ -849,4 +849,20 @@ DROP TABLE brin_hot_3; SET enable_seqscan = on; +-- Test that estimation of relation size works with tuples wider than the +-- relation fillfactor. We create a table with wide inline attributes and +-- low fillfactor, insert rows and then see how many rows EXPLAIN shows +-- before running analyze. We disable autovacuum so that it does not +-- interfere with the test. +CREATE TABLE table_fillfactor ( + n char(1000) +) with (fillfactor=10, autovacuum_enabled=off); + +INSERT INTO table_fillfactor +SELECT 'x' FROM generate_series(1,1000); + +SELECT * FROM check_estimated_rows('SELECT * FROM table_fillfactor'); + +DROP TABLE table_fillfactor; + -- End of Stats Test