petern wrote: > WITH flips(s) AS (VALUES (random()>0), (random()>0), (random()>0)) > SELECT sum(s),(SELECT sum(s) FROM flips) FROM flips; > sum(s),"(SELECT sum(s) FROM flips)" > 1,3 > --Expected output is 1,1. > > Why isn't the constant notional table table [flips] materialized just once > per CTE? > > FYI. PostgreSQL 9.6 materializes [flips] exactly once per CTE-users
Its documentation says: <https://www.postgresql.org/docs/9.6/static/queries-with.html> | 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 WITH 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 subquery. This is an implementation detail of Postgres, and it is not required by the SQL specification. SQLite chose the other side of the coin. Regards, Clemens _______________________________________________ sqlite-users mailing list [email protected] http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
