PostgreSQL 7.4.3 Documentation | ||||
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Aggregate functions compute a single result value from a set of input values. Table 9-43 shows the built-in aggregate functions. The special syntax considerations for aggregate functions are explained in Section 4.2.7. Consult Section 2.7 for additional introductory information.
Table 9-43. Aggregate Functions
It should be noted that except for count
,
these functions return a null value when no rows are selected. In
particular, sum
of no rows returns null, not
zero as one might expect. The function coalesce
may be
used to substitute zero for null when necessary.
Note: Users accustomed to working with other SQL database management systems may be surprised by the performance characteristics of certain aggregate functions in PostgreSQL when the aggregate is applied to the entire table (in other words, no WHERE clause is specified). In particular, a query like
SELECT min(col) FROM sometable;will be executed by PostgreSQL using a sequential scan of the entire table. Other database systems may optimize queries of this form to use an index on the column, if one is available. Similarly, the aggregate functions
max()
andcount()
always require a sequential scan if applied to the entire table in PostgreSQL.PostgreSQL cannot easily implement this optimization because it also allows for user-defined aggregate queries. Since
min()
,max()
, andcount()
are defined using a generic API for aggregate functions, there is no provision for special-casing the execution of these functions under certain circumstances.Fortunately, there is a simple workaround for
min()
andmax()
. The query shown below is equivalent to the query above, except that it can take advantage of a B-tree index if there is one present on the column in question.SELECT col FROM sometable ORDER BY col ASC LIMIT 1;A similar query (obtained by substituting DESC for ASC in the query above) can be used in the place of
max()
).Unfortunately, there is no similarly trivial query that can be used to improve the performance of
count()
when applied to the entire table.