Each journal materialises the results of a query within a series of fixed time windows. For example, if a query counts the numbers of rows in table, a journal can evaluate that query every hour or every day.
The duration of a time window is specified when the journal is created. This duration can be one of the standard time units - millisecond, second, minute, hour, day, week, month, quarter, year, decade, century or millennium.
Time windows are aligned with the resolution boundaries of the time unit on which they are defined. For example, each hourly interval starts on the hour while each daily interval starts at midnight. For example, creating a journal with an hourly interval will generate results for each hourly interval starting at 00:00, 01:00, 02:00 etc.
When a journal is created, a time expression is used to calculate the time window for a materialised result. This expression may be a date or time column in a table included in the query, an expression of one or more columns, or a time function.