9.21. Window Functions

Window functions provide the ability to perform calculations across sets of rows that are related to the current query row. See Section 3.5 for an introduction to this feature, and Section 4.2.8 for syntax details.

The built-in window functions are listed in Table 9.57. Note that these functions must be invoked using window function syntax, i.e., an OVER clause is required.

In addition to these functions, any built-in or user-defined general-purpose or statistical aggregate (i.e., not ordered-set or hypothetical-set aggregates) can be used as a window function; see Section 9.20 for a list of the built-in aggregates. Aggregate functions act as window functions only when an OVER clause follows the call; otherwise they act as non-window aggregates and return a single row for the entire set.

Table 9.57. General-Purpose Window Functions

FunctionReturn TypeDescription
row_number()bigintnumber of the current row within its partition, counting from 1
rank()bigintrank of the current row with gaps; same as row_number of its first peer
dense_rank()bigintrank of the current row without gaps; this function counts peer groups
percent_rank()double precisionrelative rank of the current row: (rank - 1) / (total partition rows - 1)
cume_dist()double precisioncumulative distribution: (number of partition rows preceding or peer with current row) / total partition rows
ntile(num_buckets integer)integerinteger ranging from 1 to the argument value, dividing the partition as equally as possible
lag(value anyelement [, offset integer [, default anyelement ]]) same type as value returns value evaluated at the row that is offset rows before the current row within the partition; if there is no such row, instead return default (which must be of the same type as value). Both offset and default are evaluated with respect to the current row. If omitted, offset defaults to 1 and default to null
lead(value anyelement [, offset integer [, default anyelement ]]) same type as value returns value evaluated at the row that is offset rows after the current row within the partition; if there is no such row, instead return default (which must be of the same type as value). Both offset and default are evaluated with respect to the current row. If omitted, offset defaults to 1 and default to null
first_value(value any)same type as value returns value evaluated at the row that is the first row of the window frame
last_value(value any)same type as value returns value evaluated at the row that is the last row of the window frame
nth_value(value any, nth integer) same type as value returns value evaluated at the row that is the nth row of the window frame (counting from 1); null if no such row

All of the functions listed in Table 9.57 depend on the sort ordering specified by the ORDER BY clause of the associated window definition. Rows that are not distinct when considering only the ORDER BY columns are said to be peers. The four ranking functions (including cume_dist) are defined so that they give the same answer for all peer rows.

Note that first_value, last_value, and nth_value consider only the rows within the window frame, which by default contains the rows from the start of the partition through the last peer of the current row. This is likely to give unhelpful results for last_value and sometimes also nth_value. You can redefine the frame by adding a suitable frame specification (RANGE, ROWS or GROUPS) to the OVER clause. See Section 4.2.8 for more information about frame specifications.

When an aggregate function is used as a window function, it aggregates over the rows within the current row's window frame. An aggregate used with ORDER BY and the default window frame definition produces a running sum type of behavior, which may or may not be what's wanted. To obtain aggregation over the whole partition, omit ORDER BY or use ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING. Other frame specifications can be used to obtain other effects.

Note

The SQL standard defines a RESPECT NULLS or IGNORE NULLS option for lead, lag, first_value, last_value, and nth_value. This is not implemented in PostgreSQL: the behavior is always the same as the standard's default, namely RESPECT NULLS. Likewise, the standard's FROM FIRST or FROM LAST option for nth_value is not implemented: only the default FROM FIRST behavior is supported. (You can achieve the result of FROM LAST by reversing the ORDER BY ordering.)

cume_dist computes the fraction of partition rows that are less than or equal to the current row and its peers, while percent_rank computes the fraction of partition rows that are less than the current row, assuming the current row does not exist in the partition.