As we saw in the previous section, the query planner needs to estimate the number of rows retrieved by a query in order to make good choices of query plans. This section provides a quick look at the statistics that the system uses for these estimates.
One component of the statistics is the total number of entries in each table and index, as well as the number of disk blocks occupied by each table and index. This information is kept in the table pg_class
, in the columns reltuples
and relpages
. We can look at it with queries similar to this one:
SELECT relname, relkind, reltuples, relpages FROM pg_class WHERE relname LIKE 'tenk1%'; relname | relkind | reltuples | relpages ----------------------+---------+-----------+---------- tenk1 | r | 10000 | 358 tenk1_hundred | i | 10000 | 30 tenk1_thous_tenthous | i | 10000 | 30 tenk1_unique1 | i | 10000 | 30 tenk1_unique2 | i | 10000 | 30 (5 rows)
Here we can see that tenk1
contains 10000 rows, as do its indexes, but the indexes are (unsurprisingly) much smaller than the table.
For efficiency reasons, reltuples
and relpages
are not updated on-the-fly, and so they usually contain somewhat out-of-date values. They are updated by VACUUM
, ANALYZE
, and a few DDL commands such as CREATE INDEX
. A VACUUM
or ANALYZE
operation that does not scan the entire table (which is commonly the case) will incrementally update the reltuples
count on the basis of the part of the table it did scan, resulting in an approximate value. In any case, the planner will scale the values it finds in pg_class
to match the current physical table size, thus obtaining a closer approximation.
Most queries retrieve only a fraction of the rows in a table, due to WHERE
clauses that restrict the rows to be examined. The planner thus needs to make an estimate of the selectivity of WHERE
clauses, that is, the fraction of rows that match each condition in the WHERE
clause. The information used for this task is stored in the pg_statistic
system catalog. Entries in pg_statistic
are updated by the ANALYZE
and VACUUM ANALYZE
commands, and are always approximate even when freshly updated.
Rather than look at pg_statistic
directly, it's better to look at its view pg_stats
when examining the statistics manually. pg_stats
is designed to be more easily readable. Furthermore, pg_stats
is readable by all, whereas pg_statistic
is only readable by a superuser. (This prevents unprivileged users from learning something about the contents of other people's tables from the statistics. The pg_stats
view is restricted to show only rows about tables that the current user can read.) For example, we might do:
SELECT attname, inherited, n_distinct, array_to_string(most_common_vals, E'\n') as most_common_vals FROM pg_stats WHERE tablename = 'road'; attname | inherited | n_distinct | most_common_vals ---------+-----------+------------+------------------------------------ name | f | -0.363388 | I- 580 Ramp+ | | | I- 880 Ramp+ | | | Sp Railroad + | | | I- 580 + | | | I- 680 Ramp name | t | -0.284859 | I- 880 Ramp+ | | | I- 580 Ramp+ | | | I- 680 Ramp+ | | | I- 580 + | | | State Hwy 13 Ramp (2 rows)
Note that two rows are displayed for the same column, one corresponding to the complete inheritance hierarchy starting at the road
table (inherited
=t
), and another one including only the road
table itself (inherited
=f
).
The amount of information stored in pg_statistic
by ANALYZE
, in particular the maximum number of entries in the most_common_vals
and histogram_bounds
arrays for each column, can be set on a column-by-column basis using the ALTER TABLE SET STATISTICS
command, or globally by setting the default_statistics_target configuration variable. The default limit is presently 100 entries. Raising the limit might allow more accurate planner estimates to be made, particularly for columns with irregular data distributions, at the price of consuming more space in pg_statistic
and slightly more time to compute the estimates. Conversely, a lower limit might be sufficient for columns with simple data distributions.
Further details about the planner's use of statistics can be found in Chapter 67.
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