OVER, PARTITION BY, and WINDOW
Window functions were introduced in the SQL:2003 standard and expanded in the SQL:2011 standard. They let you run calculations on a set of table rows that are related to the current row in some way.
Unlike aggregate functions, window functions don't group rows into one output row: the number of rows in the resulting table is always the same as in the source table.
If a query contains both aggregate and window functions, grouping is performed and aggregate function values are calculated first. The calculated values of aggregate functions can be used as window function arguments (but not the other way around).
General syntax for calling a window function is as follows
function_name([expression [, expression ...]]) OVER (window_definition) or function_name([expression [, expression ...]]) OVER window_name
window_name (window name) is an arbitrary ID that is unique within the query and
expression is an arbitrary expression that contains no window function calls.
In the query, each window name must be mapped to the window definition (
SELECT F0(...) OVER (window_definition_0), F1(...) OVER w1, F2(...) OVER w2, ... FROM my_table WINDOW w1 AS (window_definition_1), ... w2 AS (window_definition_2) ;
window_definition is written as
[ PARTITION BY (expression AS column_identifier | column_identifier) [, ...] ] [ ORDER BY expression [ASC | DESC] ] [ frame_definition ]
You can set an optional frame definition (
frame_definition) one of two ways:
ROWS BETWEEN frame_begin AND frame_end
The frame start (
frame_begin) and frame end (
frame_end) are set one of the following ways:
Here, the frame
offset is a non-negative numeric literal. If the frame end isn't set, the
CURRENT ROW is assumed.
There should be no window function calls in any of the expressions inside the window definition.
PARTITION BY is set, the source table rows are grouped into partitions, which are then handled independently of each other.
PARTITION BY isn't set, all rows in the source table are put in the same partition. If
ORDER BY is set, it determines the order of rows in a partition.
PARTITION BY and GROUP BY you can use aliases and SessionWindow.
ORDER BY is omitted, the order of rows in the partition is undefined.
frame_definition specifies a set of partition rows that fall into the window frame associated with the current row.
ROWS mode (the only one that YQL currently supports), the window frame contains rows with the specified offsets relative to the current row in the partition.
- For example, if
ROWS BETWEEN 3 PRECEDING AND 5 FOLLOWINGis used, the window frame contains 3 rows preceding the current one, the current row, and 5 rows following it.
The set of rows in the window frame may change depending on which row is the current one.
- For example, for the first row in the partition, the
ROWS BETWEEN 3 PRECEDING AND 1 PRECEDINGwindow frame will have no rows.
UNBOUNDED PRECEDING as the frame start means "from the first partition row" and
UNBOUNDED FOLLOWING as the frame end — "up to the last partition row". Setting
CURRENT ROW means "from/to the current row".
frame_definition is specified, a set of rows to be included in the window frame depends on whether there is
ORDER BY in the
Namely, if there is
ORDER BY, then
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW is implicitly assumed. If none, then
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.
Further, depending on the specific window function, it's calculated either based on the set of rows in the partition or the set of rows in the window frame.
SELECT COUNT(*) OVER w AS rows_count_in_window, some_other_value -- access the current row FROM `my_table` WINDOW w AS ( PARTITION BY partition_key_column ORDER BY int_column );
SELECT LAG(my_column, 2) OVER w AS row_before_previous_one FROM `my_table` WINDOW w AS ( PARTITION BY partition_key_column );
SELECT -- AVG (like all aggregate functions used as window functions) -- is calculated on the window frame AVG(some_value) OVER w AS avg_of_prev_current_next, some_other_value -- access the current row FROM my_table WINDOW w AS ( PARTITION BY partition_key_column ORDER BY int_column ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING );
SELECT -- LAG doesn't depend on the window frame position LAG(my_column, 2) OVER w AS row_before_previous_one FROM my_table WINDOW w AS ( PARTITION BY partition_key_column ORDER BY my_column );
Functions calculated on the
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWINGor
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROWwindow frame are implemented efficiently (do not require additional memory and their computation runs on a partition in O(partition size) time).
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWINGwindow frame, you can choose the execution strategy in RAM by specifying the
COMPACThint after the
PARTITION COMPACT BY keyor
PARTITION COMPACT BY ()(if
PARTITION BYwas missing initially).
COMPACThint is specified, this requires additional memory equal to O(partition size), but then no extra
JOINoperation is made.
If the window frame doesn't start with
UNBOUNDED PRECEDING, calculating window functions on this window requires additional memory equal to O(the maximum number of rows from the window boundaries to the current row), while the computation time is equal to O(number_of_partition_rows * window_size).
For the window frame starting with
UNBOUNDED PRECEDINGand ending with
UNBOUNDED FOLLOWING, additional memory equal to O(N) is required and the computation time is equal to O(N * number_of_partition_rows).
LAG(expr, N)functions always require O(N) of RAM.
Given the above, a query with
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING should, if possible, be changed to
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW by reversing the
ORDER BY sorting order.