Due to the introduction of null checks in map, flatMap, and exists, in
FlattenOptionOperation in order to resolve #1053, as well as to support non-ansi
compliant string concatenation as outlined in #1295, large conditional composites
became common. For example:
case class Holder(value:Option[String])
// The following statement
query[Holder].map(h => h.value.map(_ + "foo"))
// Will yield the following result
SELECT CASE WHEN h.value IS NOT NULL THEN h.value || 'foo' ELSE null END FROM Holder h
Now, let's add a getOrElse statement to the clause that requires an additional
wrapped null check. We cannot rely on there being a map call beforehand
since we could be reading value as a nullable field directly from the database).
// The following statement
query[Holder].map(h => h.value.map(_ + "foo").getOrElse("bar"))
// Yields the following result:
SELECT CASE WHEN
CASE WHEN h.value IS NOT NULL THEN h.value || 'foo' ELSE null END
IS NOT NULL THEN
CASE WHEN h.value IS NOT NULL THEN h.value || 'foo' ELSE null END
ELSE 'bar' END FROM Holder h
This of course is highly redundant and can be reduced to simply:
SELECT CASE WHEN h.value IS NOT NULL AND (h.value || 'foo') IS NOT NULL THEN h.value || 'foo' ELSE 'bar' END FROM Holder h
This reduction is done by the "Center Rule." There are some other simplification
rules as well. Note how we are force to null-check both h.value as well as (h.value || 'foo') because
a user may use Option[T].flatMap and explicitly transform a particular value to null.
Due to the introduction of null checks in
map,flatMap, andexists, inFlattenOptionOperationin order to resolve #1053, as well as to support non-ansi compliant string concatenation as outlined in #1295, large conditional composites became common. For example:getOrElsestatement to the clause that requires an additional wrapped null check. We cannot rely on there being amapcall beforehand since we could be readingvalueas a nullable field directly from the database).// The following statement query[Holder].map(h => h.value.map(_ + "foo").getOrElse("bar")) // Yields the following result: SELECT CASE WHEN CASE WHEN h.value IS NOT NULL THEN h.value || 'foo' ELSE null END IS NOT NULL THEN CASE WHEN h.value IS NOT NULL THEN h.value || 'foo' ELSE null END ELSE 'bar' END FROM Holder hh.valueas well as(h.value || 'foo')because a user may useOption[T].flatMapand explicitly transform a particular value tonull.