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and so they usually contain somewhat out-of-date values.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 61, How the Planner Uses Statistics.