Fix round double for postgres

sqlglot/dialects/postgres.py

@@ -265,6 +265,30 @@ def _versioned_anyvalue_sql(self: Postgres.Generator, expression: exp.AnyValue)
     return rename_func("ANY_VALUE")(self, expression)
 
 
+def _to_decimal(self: Postgres.Generator, expression: exp.Expression) -> exp.Expression:
+    if not expression.type:
+        from sqlglot.optimizer.annotate_types import annotate_types
+
+        annotate_types(expression, dialect=self.dialect)
+
+    if expression.type and expression.type == exp.DataType.build("DOUBLE"):
+        return exp.cast(expression, to=exp.DataType.build("DECIMAL"))
+    return expression
+
+
+def _round(self: Postgres.Generator, expression: exp.Round) -> str:
+    # ROUND(double precision, integer) is not permitted in Postgres
+    # so it's necessary to cast double precision to decimal before rounding.
+
+    this = self.sql(expression, "this")
+    decimals = self.sql(expression, "decimals")
+
+    if not decimals:
+        return self.func("ROUND", this)
+
+    return self.func("ROUND", _to_decimal(self, expression.this), decimals)
+
+
 class Postgres(Dialect):
     INDEX_OFFSET = 1
     TYPED_DIVISION = True
@@ -613,6 +637,7 @@ class Generator(generator.Generator):
             exp.Rand: rename_func("RANDOM"),
             exp.RegexpLike: lambda self, e: self.binary(e, "~"),
             exp.RegexpILike: lambda self, e: self.binary(e, "~*"),
+            exp.Round: _round,
             exp.Select: transforms.preprocess(
                 [
                     transforms.eliminate_semi_and_anti_joins,

tests/dialects/test_postgres.py

@@ -1460,3 +1460,24 @@ def _validate_udt(sql: str):
         _validate_udt('CAST(5 AS "MySchema"."MyType")')
         _validate_udt('CAST(5 AS MySchema."MyType")')
         _validate_udt('CAST(5 AS "MyCatalog"."MySchema"."MyType")')
+
+    def test_round(self):
+        self.validate_identity("ROUND(x)")
+        self.validate_identity("ROUND(CAST(x AS DOUBLE PRECISION))")
+        self.validate_identity("ROUND(CAST(x AS DECIMAL), 4)")
+        self.validate_identity("ROUND(CAST(x AS INT), 4)")
+        self.validate_identity(
+            "ROUND(x, y)",
+            "ROUND(CAST(x AS DECIMAL), y)",
+        )
+        self.validate_all(
+            "ROUND(CAST(CAST(x AS DOUBLE PRECISION) AS DECIMAL), 4)",
+            read={
+                "postgres": "ROUND(x::DOUBLE, 4)",
+                "hive": "ROUND(x::DOUBLE, 4)",
+                "bigquery": "ROUND(x::DOUBLE, 4)",
+            },
+        )
+        self.validate_all(
+            "ROUND(CAST(x AS DECIMAL(18, 3)), 4)", read={"duckdb": "ROUND(x::DECIMAL, 4)"}
+        )