fix(snowflake): manually construct quantile calls with WITHIN GROUP (…

…#8846)

ibis/backends/snowflake/compiler.py

@@ -453,7 +453,20 @@ def visit_Quantile(self, op, *, arg, quantile, where):
         # the constant into an expression
         if where is not None:
             arg = self.if_(where, arg, NULL)
-        return self.f.percentile_cont(arg, quantile)
+
+        # The Snowflake SQLGlot dialect rewrites calls to `percentile_cont` to
+        # include     WITHIN GROUP (ORDER BY ...)
+        # as per https://docs.snowflake.com/en/sql-reference/functions/percentile_cont
+        # using the rule `add_within_group_for_percentiles`
+        #
+        # If we have copy=False set in our call to `compile`, if there is more
+        # than one quantile, the rewrite rule fails on the second pass because
+        # of some mutation in the first pass. To avoid this error, we create the
+        # expression with the within group included already and skip the (now
+        # unneeded) rewrite rule.
+        order_by = sge.Order(expressions=[sge.Ordered(this=arg)])
+        quantile = self.f.percentile_cont(quantile)
+        return sge.WithinGroup(this=quantile, expression=order_by)
 
     def visit_CountStar(self, op, *, arg, where):
         if where is None:

ibis/backends/snowflake/tests/snapshots/test_compiler/test_more_than_one_quantile/two_quantiles.sql

@@ -0,0 +1,6 @@
+SELECT
+  PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY
+    "t0"."ROW_COUNT") AS "quantile_0_25",
+  PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY
+    "t0"."ROW_COUNT") AS "quantile_0_75"
+FROM "t" AS "t0"

ibis/backends/snowflake/tests/test_compiler.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+import ibis
+from ibis import _
+
+
+def test_more_than_one_quantile(snapshot):
+    tables = ibis.table(name="t", schema={"ROW_COUNT": "int"})
+
+    expr = tables.aggregate(
+        quantile_0_25=_.ROW_COUNT.quantile(0.25),
+        quantile_0_75=_.ROW_COUNT.quantile(0.75),
+    )
+
+    sql = ibis.to_sql(expr, dialect="snowflake")
+    snapshot.assert_match(sql, "two_quantiles.sql")