This works because PostgreSQL ‘s implementation evaluates only as many rows of a Having 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 Which have query’s output anyway.
A useful property of With 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 That have queries. Thus, expensive calculations that are needed in multiple places can be placed within a With 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 That have 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 Having being used with Find, but it can be attached in the same way to , Modify, or Erase. In each case it effectively provides temporary table(s) that can be referred to in the main command.
You can use data-modifying statements (, Enhance, or Erase) in Having. This allows you to perform several different operations in the same query. An example is:
This query effectively moves rows from issues to products_diary. The Remove in With deletes the specified rows from products, returning their contents by means of its Returning clause; and then the primary query reads that output and inserts it into products_record. Leer más
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