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 9-56. Note that these functions must be invoked using window function syntax; that is an OVER clause is required.
In addition to these functions, any built-in or user-defined normal aggregate function (but 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 regular aggregates.
Table 9-56. General-Purpose Window Functions
| Function | Return Type | Description |
|---|---|---|
row_number() | bigint | number of the current row within its partition, counting from 1 |
rank() | bigint | rank of the current row with gaps; same as row_number of its first peer |
dense_rank() | bigint | rank of the current row without gaps; this function counts peer groups |
percent_rank() | double precision | relative rank of the current row: (rank - 1) / (total rows - 1) |
cume_dist() | double precision | relative rank of the current row: (number of rows preceding or peer with current row) / (total rows) |
ntile( | integer | integer ranging from 1 to the argument value, dividing the partition as equally as possible |
lag( | same type as | 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( | same type as | 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( | same type as | returns value evaluated at the row that is the first row of the window frame |
last_value( | same type as | returns value evaluated at the row that is the last row of the window frame |
nth_value( | same type as | 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 9-56 depend on the sort ordering specified by the ORDER BY clause of the associated window definition. Rows that are not distinct in the ORDER BY ordering are said to be peers; the four ranking functions are defined so that they give the same answer for any two 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 or ROWS) 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 NULLSorIGNORE NULLSoption forlead,lag,first_value,last_value, andnth_value. This is not implemented in PostgreSQL: the behavior is always the same as the standard's default, namelyRESPECT NULLS. Likewise, the standard'sFROM FIRSTorFROM LASToption fornth_valueis not implemented: only the defaultFROM FIRSTbehavior is supported. (You can achieve the result ofFROM LASTby reversing theORDER BYordering.)
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