feat(postgres): Support generation of exp.CountIf (#4709)

* feat(postgres): Support generation of exp.CountIf

* Switch to dialect.py impl

Author: VaggelisD

sqlglot/dialects/postgres.py

@@ -33,6 +33,7 @@
     trim_sql,
     ts_or_ds_add_cast,
     strposition_sql,
+    count_if_to_sum,
 )
 from sqlglot.generator import unsupported_args
 from sqlglot.helper import is_int, seq_get
@@ -621,6 +622,7 @@ class Generator(generator.Generator):
             exp.Levenshtein: _levenshtein_sql,
             exp.JSONObjectAgg: rename_func("JSON_OBJECT_AGG"),
             exp.JSONBObjectAgg: rename_func("JSONB_OBJECT_AGG"),
+            exp.CountIf: count_if_to_sum,
         }
 
         TRANSFORMS.pop(exp.CommentColumnConstraint)

tests/dialects/test_dialect.py

@@ -2633,6 +2633,8 @@ def test_count_if(self):
                 "snowflake": "SELECT COUNT_IF(col % 2 = 0) FROM foo",
                 "sqlite": "SELECT SUM(IIF(col % 2 = 0, 1, 0)) FROM foo",
                 "tsql": "SELECT COUNT_IF(col % 2 = 0) FROM foo",
+                "postgres": "SELECT SUM(CASE WHEN col % 2 = 0 THEN 1 ELSE 0 END) FROM foo",
+                "redshift": "SELECT SUM(CASE WHEN col % 2 = 0 THEN 1 ELSE 0 END) FROM foo",
             },
         )
         self.validate_all(