Window definition syntax

partitionByExpr	Optionally specifies an EPL expression that should involve the input item in some way and that returns a comparable type. A partition by clause effectively creates a separate window for each encountered distinct value of partitionByExpr.
windowDurationExpr	Specifies a float expression that indicates a duration of a number of seconds. The window contains the items received within the last windowDurationExpr seconds. See Defining time-based windows.
batchPeriodExpr	Specifies a float expression that indicates an interval period of a number of seconds. The window updates its contents every batchPeriodExpr seconds. See Defining batched windows.
windowSizeExpr	Specifies an integer expression that indicates the number of items you want to retain in the window. The window contains the most recent windowSizeExpr items. See Defining size-based windows.
keyExpr	Specifies an EPL expression that must contain at least one reference to the input item and must return a comparable type. See Comparable types in the "Types" section of the Apama EPL Reference. If you add a with unique clause, if there is more than one item in the window that has the same value for the key identified by keyExpr, only the most recently received item is considered to be in the window. See Defining content-dependent windows.
batchSizeExpr	Specifies an integer expression that indicates a number of items. The window updates its contents after every batchSizeExpr items that match the query are found. See Defining batched windows.

The window definition is optional in a stream query. If you do not specify any window then, for any given activation of the stream query, the stream query operates on only the items that are current for that activation. Typically this is a single event. However, if the source for this query is, for example, a stream query with a batched window then the items in each batch will be processed together as in the following example:

The simplest window is one that contains all items that have ever been in the stream. The corresponding window definition is retain all. Conceptually, once an item enters a retain all window, it remains in the window indefinitely (or until the stream query is terminated). The following query evaluates the running mean of all items that have ever been in the values stream:

When you use a custom (user-defined) aggregate function in a query that contains an unbounded window, you cannot also use a bounded aggregate function. You should also be aware that, if you use a badly implemented custom aggregate function in a query that contains an unbounded window, then this can result in uncontrolled memory usage. See Defining custom aggregate functions.