This works because PostgreSQL ‘s implementation evaluates only as many rows of a With query as are actually fetched by the parent query. Using this trick in production is not recommended, because other systems might work differently. Also, it usually won’t work if you make the outer query sort the recursive query’s results or join them to some other table, because in such cases the outer query will usually try to fetch all of the Having query’s output anyway.
A useful property of That have queries is that they are evaluated only once per execution of the parent query, even if they are referred to more than once by the parent query or sibling With queries. Thus, expensive calculations that are needed in multiple places can be placed within a That have query to avoid redundant work. Another possible application is to prevent unwanted multiple evaluations of functions with side-effects. However, the other side of this coin is that the optimizer is less able to push restrictions from the parent query down into a With query than an ordinary sub-query. The Having query will generally be evaluated as written, without suppression of rows that the parent query might discard afterwards. (But, as mentioned above, evaluation might stop early if the reference(s) to the query demand only a limited number of rows.)
The examples above only show With being used with Pick, but it can be attached in the same way to , Revise, or Remove. In each case it effectively provides temporary table(s) that can be referred to in the main command.
eight.8.2. Data-Changing Statements in the With
You can use data-modifying statements (, Change, or Remove) in With. This allows you to perform several different operations in the same query. An example is:
This query effectively moves rows from situations to products_journal. (más…)