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 With 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 Which 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 That have 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 Which have being used with Discover, 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.
seven.8.2. Data-Altering Comments inside Which have
You can use data-modifying statements (, Upgrade, or Erase) in Which have. This allows you to perform several different operations in the same query. An example is:
This query effectively moves rows from circumstances to products_journal. The Delete in That have deletes the specified rows from products, returning their contents by means of its Coming back clause; and then the primary query reads that output and inserts it into products_record.
A fine point of the above example is that the Having clause is attached to the , not the sub-Pick within the . This is necessary because data-modifying statements are only allowed in Having clauses that are attached to the top-level statement. However, normal That have visibility rules apply, so it is possible to refer to the With statement’s output from the sub-Pick.
Data-modifying statements in Which have usually have Coming back clauses, as seen in the example above. It is the output of the Returning clause, not the target table of the data-modifying statement, that forms the temporary table that can be referred to by the rest of the query. If a data-modifying statement in With lacks a Returning clause, then it forms no temporary table and cannot be referred to in the rest of the query. Such a statement will be executed nonetheless. A not-particularly-useful example is:
This example would remove all rows from tables foo and club. The number of affected rows reported to the client would only include rows removed from bar.
Recursive self-references in data-modifying statements are not allowed. In some cases it is possible to work around this limitation by referring to the output of a recursive With, for example:
Data-modifying statements in Which have are executed exactly once, and always to completion, independently of whether the primary query reads all (or indeed any) of their output. Notice that this is different from the rule for Get a hold of in With: as stated in the previous section, execution of a Pick is carried only as far as the primary query demands its output.