This works because PostgreSQL ‘s implementation evaluates only as many rows of a That have 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 Having 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 Having queries. Thus, expensive calculations that are needed in multiple places can be placed within a Having 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 With 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 That have being used with Get a hold of, but it can be attached in the same way to , Update, or Erase. In each case it effectively provides temporary table(s) that can be referred to in the main command.
seven.8.dos. Data-Modifying Comments within the With
You can use data-modifying statements (, Improve, or Erase) in With. This allows you to perform several different operations in the same query. An example is:
This query effectively moves rows from points to products_record. The Erase 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_diary.
A fine point of the above example is that the Which have clause is attached to the , not the sub-Find within the . This is necessary because data-modifying statements are only allowed in Which have clauses that are attached to the top-level statement. However, normal Which have visibility rules apply, so it is possible to refer to the Which have statement’s output from the sub-Pick.
Data-modifying statements in With usually have Coming back clauses, as seen in the example above. It is the output of the Coming back 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 Which have lacks a Coming back 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 bar. 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 That have, for example:
Data-modifying statements in With 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 Look for in With: as stated in the previous section, execution of a Get a hold of is carried only as far as the primary query demands its output.