| Postgres-XC 1.0.4 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Chapter 9. Functions and Operators | Fast Forward | Next |
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.
The built-in window functions are listed in Table 9-44. 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 aggregate function can be used as a window function (see Section 9.18 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 9-44. 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(num_buckets integer)
| integer | integer ranging from 1 to the argument value, dividing the partition as equally as possible |
lag(value any
[, offset integer
[, default any ]])
| 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. Both offset and default are evaluated with respect to the current row. If omitted, offset defaults to 1 and default to null |
lead(value any
[, offset integer
[, default any ]])
| 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. 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 9-44 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 NULLS or IGNORE NULLS option for
lead,lag,first_value,last_value, andnth_value. This is not implemented in Postgres-XC: the behavior is always the same as the standard's default, namely RESPECT NULLS. Likewise, the standard's FROM FIRST or FROM LAST option fornth_valueis not implemented: only the default FROM FIRST behavior is supported. (You can achieve the result of FROM LAST by reversing the ORDER BY ordering.)