Database.Esqueleto.Experimental.From.CommonTableExpression

WITH clause used to introduce a Common Table Expression (CTE). CTEs are supported in most modern SQL engines and can be useful in performance tuning. In Esqueleto, CTEs should be used as a subquery memoization tactic. When writing plain SQL, CTEs are sometimes used to organize the SQL code, in Esqueleto, this is better achieved through function that return SqlQuery values.

select $ do
cte <- with subQuery
cteResult <- from cte
where_ $ cteResult ...
pure cteResult

WARNING: In some SQL engines using a CTE can diminish performance. In these engines the CTE is treated as an optimization fence. You should always verify that using a CTE will in fact improve your performance over a regular subquery.

Notably, in PostgreSQL prior to version 12, CTEs are always fully calculated, which can potentially significantly pessimize queries. As of PostgreSQL 12, non-recursive and side-effect-free queries may be inlined and optimized accordingly if not declared MATERIALIZED to get the previous behaviour. See the PostgreSQL CTE documentation, section Materialization, for more information. To use a MATERIALIZED query in Esquelto, see functions withMaterialized and withRecursiveMaterialized.

Since: 3.4.0.0

WITH RECURSIVE allows one to make a recursive subquery, which can reference itself. Like WITH, this is supported in most modern SQL engines. Useful for hierarchical, self-referential data, like a tree of data.

select $ do
cte <- withRecursive
         (do
             person <- from $ table @Person
             where_ $ person ^. PersonId ==. val personId
             pure person
         )
         unionAll_
         (\self -> do
             (p :& f :& p2 :& pSelf) <- from self
                      `innerJoin` $ table @Follow
                      `on` (\(p :& f) ->
                              p ^. PersonId ==. f ^. FollowFollower)
                      `innerJoin` $ table @Person
                      `on` (\(p :& f :& p2) ->
                              f ^. FollowFollowed ==. p2 ^. PersonId)
                      `leftJoin` self
                      `on` (\(_ :& _ :& p2 :& pSelf) ->
                              just (p2 ^. PersonId) ==. pSelf ?. PersonId)
             where_ $ isNothing (pSelf ?. PersonId)
             groupBy (p2 ^. PersonId)
             pure p2
         )
from cte

Since: 3.4.0.0