Fix(redshift)!: turn off multi-arg coalesce simplification (#4877)

Author: georgesittas

sqlglot/optimizer/simplify.py

@@ -117,7 +117,7 @@ def _simplify(expression, root=True):
         node = flatten(node)
         node = simplify_connectors(node, root)
         node = remove_complements(node, root)
-        node = simplify_coalesce(node)
+        node = simplify_coalesce(node, dialect)
         node.parent = expression.parent
         node = simplify_literals(node, root)
         node = simplify_equality(node)
@@ -775,7 +775,7 @@ def _is_constant(expression: exp.Expression) -> bool:
     return isinstance(expression, exp.CONSTANTS) or _is_date_literal(expression)
 
 
-def simplify_coalesce(expression):
+def simplify_coalesce(expression: exp.Expression, dialect: DialectType) -> exp.Expression:
     # COALESCE(x) -> x
     if (
         isinstance(expression, exp.Coalesce)
@@ -785,6 +785,12 @@ def simplify_coalesce(expression):
     ):
         return expression.this
 
+    # We can't convert `COALESCE(x, 1) = 2` into `NOT x IS NULL AND x = 2` for redshift,
+    # because they are not always equivalent. For example,  if `x` is `NULL` and it comes
+    # from a table, then the result is `NULL`, despite `FALSE AND NULL` evaluating to `FALSE`
+    if dialect == "redshift":
+        return expression
+
     if not isinstance(expression, COMPARISONS):
         return expression
 

tests/fixtures/optimizer/simplify.sql

@@ -843,6 +843,10 @@ x;
 COALESCE(x, 1) = 2;
 NOT x IS NULL AND x = 2;
 
+# dialect: redshift
+COALESCE(x, 1) = 2;
+COALESCE(x, 1) = 2;
+
 2 = COALESCE(x, 1);
 NOT x IS NULL AND x = 2;