Feat: transpile BQ APPROX_QUANTILES to DuckDB (#6349)

* fix bq approx_quantiles parsing

* add duckdb transpilation

* first arg should use _parse_disjunction

* num quantiles arg should use _parse_bitwise

* pass array of quantiles

* remove BQ parser, handle distinct in duck generator

* pr feedback

* Update sqlglot/dialects/duckdb.py

---------

Co-authored-by: Jo <[email protected]>

Author: treysp

sqlglot/dialects/duckdb.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from decimal import Decimal
 from itertools import groupby
 import re
 import typing as t
@@ -1402,7 +1403,7 @@ def ignorenulls_sql(self, expression: exp.IgnoreNulls) -> str:
             if isinstance(this, exp.First):
                 this = exp.AnyValue(this=this.this)
 
-            if not isinstance(this, exp.AnyValue):
+            if not isinstance(this, (exp.AnyValue, exp.ApproxQuantiles)):
                 self.unsupported("IGNORE NULLS is not supported for non-window functions.")
 
             return self.sql(this)
@@ -1594,3 +1595,40 @@ def round_sql(self, expression: exp.Round) -> str:
                     truncate = None
 
             return self.func(func, this, decimals, truncate)
+
+        def approxquantiles_sql(self, expression: exp.ApproxQuantiles) -> str:
+            """
+            BigQuery's APPROX_QUANTILES(expr, n) returns an array of n+1 approximate quantile values
+            dividing the input distribution into n equal-sized buckets.
+
+            Both BigQuery and DuckDB use approximate algorithms for quantile estimation, but BigQuery
+            does not document the specific algorithm used so results may differ. DuckDB does not
+            support RESPECT NULLS.
+            """
+            this = expression.this
+            if isinstance(this, exp.Distinct):
+                # APPROX_QUANTILES requires 2 args and DISTINCT node grabs both
+                if len(this.expressions) < 2:
+                    self.unsupported("APPROX_QUANTILES requires a bucket count argument")
+                    return self.function_fallback_sql(expression)
+                num_quantiles_expr = this.expressions[1].pop()
+            else:
+                num_quantiles_expr = expression.expression
+
+            if not isinstance(num_quantiles_expr, exp.Literal) or not num_quantiles_expr.is_int:
+                self.unsupported("APPROX_QUANTILES bucket count must be a positive integer")
+                return self.function_fallback_sql(expression)
+
+            num_quantiles = t.cast(int, num_quantiles_expr.to_py())
+            if num_quantiles <= 0:
+                self.unsupported("APPROX_QUANTILES bucket count must be a positive integer")
+                return self.function_fallback_sql(expression)
+
+            quantiles = [
+                exp.Literal.number(Decimal(i) / Decimal(num_quantiles))
+                for i in range(num_quantiles + 1)
+            ]
+
+            return self.sql(
+                exp.ApproxQuantile(this=this, quantile=exp.Array(expressions=quantiles))
+            )

tests/dialects/test_bigquery.py

@@ -3451,3 +3451,93 @@ def test_round(self):
                 "duckdb": "SELECT ROUND_EVEN(CAST('2.25' AS DECIMAL), 1) AS value",
             },
         )
+
+    def test_approx_quantiles(self):
+        self.validate_identity("APPROX_QUANTILES(x, 2)")
+        self.validate_identity("APPROX_QUANTILES(FALSE OR TRUE, 2)")
+        self.validate_identity("APPROX_QUANTILES((SELECT 1 AS val), CAST(2.1 AS INT64))")
+        self.validate_identity("APPROX_QUANTILES(DISTINCT x, 2)")
+        self.validate_identity("APPROX_QUANTILES(x, 2 RESPECT NULLS)")
+        self.validate_identity("APPROX_QUANTILES(x, 2 IGNORE NULLS)")
+        self.validate_identity("APPROX_QUANTILES(DISTINCT x, 2 RESPECT NULLS)")
+
+    def test_approx_quantiles_to_duckdb(self):
+        self.validate_all(
+            "APPROX_QUANTILES(x, 1)",
+            write={"duckdb": "APPROX_QUANTILE(x, [0, 1])"},
+        )
+        self.validate_all(
+            "APPROX_QUANTILES(x, 2)",
+            write={"duckdb": "APPROX_QUANTILE(x, [0, 0.5, 1])"},
+        )
+        self.validate_all(
+            "APPROX_QUANTILES(x, 4)",
+            write={"duckdb": "APPROX_QUANTILE(x, [0, 0.25, 0.5, 0.75, 1])"},
+        )
+        self.validate_all(
+            "APPROX_QUANTILES(DISTINCT x, 2)",
+            write={"duckdb": "APPROX_QUANTILE(DISTINCT x, [0, 0.5, 1])"},
+        )
+
+        with self.subTest("APPROX_QUANTILES 100 buckets"):
+            result = self.parse_one("APPROX_QUANTILES(x, 100)").sql("duckdb")
+            self.assertEqual(result.count("APPROX_QUANTILE("), 1)
+            self.assertIn("0.01", result)
+            self.assertIn("0.99", result)
+            self.assertRegex(result, r"APPROX_QUANTILE\(x, \[.*\]\)")
+
+        for expr in ("x + y", "CASE WHEN x > 0 THEN x ELSE 0 END", "ABS(x)"):
+            with self.subTest(expr=expr):
+                self.validate_all(
+                    f"APPROX_QUANTILES({expr}, 2)",
+                    write={"duckdb": f"APPROX_QUANTILE({expr}, [0, 0.5, 1])"},
+                )
+
+        with self.subTest("non-literal bucket count"):
+            with self.assertRaises(UnsupportedError):
+                self.parse_one("APPROX_QUANTILES(x, bucket_count)").sql(
+                    "duckdb", unsupported_level=ErrorLevel.RAISE
+                )
+
+        with self.subTest("non-integer bucket count"):
+            for value in ("0", "-1", "2.5"):
+                with self.subTest(value=value):
+                    with self.assertRaises(UnsupportedError):
+                        self.parse_one(f"APPROX_QUANTILES(x, {value})").sql(
+                            "duckdb", unsupported_level=ErrorLevel.RAISE
+                        )
+
+        with self.subTest("NULL bucket count"):
+            with self.assertRaises(UnsupportedError):
+                self.parse_one("APPROX_QUANTILES(x, NULL)").sql(
+                    "duckdb", unsupported_level=ErrorLevel.RAISE
+                )
+
+        with self.subTest("missing bucket count"):
+            with self.assertRaises(UnsupportedError):
+                self.parse_one("APPROX_QUANTILES(x)").sql(
+                    "duckdb", unsupported_level=ErrorLevel.RAISE
+                )
+
+        with self.subTest("missing bucket count with DISTINCT"):
+            with self.assertRaises(UnsupportedError):
+                self.parse_one("APPROX_QUANTILES(DISTINCT x)").sql(
+                    "duckdb", unsupported_level=ErrorLevel.RAISE
+                )
+
+        with self.subTest("APPROX_QUANTILES IGNORE NULLS"):
+            # No warning: IGNORE NULLS is the default behavior in DuckDB
+            from sqlglot.generator import logger as generator_logger
+
+            with mock.patch.object(generator_logger, "warning") as mock_warning:
+                self.validate_all(
+                    "APPROX_QUANTILES(x, 2 IGNORE NULLS)",
+                    write={"duckdb": "APPROX_QUANTILE(x, [0, 0.5, 1])"},
+                )
+                mock_warning.assert_not_called()
+
+        with self.subTest("APPROX_QUANTILES RESPECT NULLS"):
+            with self.assertRaises(UnsupportedError):
+                self.parse_one("APPROX_QUANTILES(x, 2 RESPECT NULLS)").sql(
+                    "duckdb", unsupported_level=ErrorLevel.RAISE
+                )