chore: Migrate fillna_op operator to SQLGlot

Author: jialuoo

bigframes/core/compile/sqlglot/expressions/generic_ops.py

@@ -24,6 +24,7 @@
 import bigframes.core.compile.sqlglot.scalar_compiler as scalar_compiler
 
 register_unary_op = scalar_compiler.scalar_op_compiler.register_unary_op
+register_binary_op = scalar_compiler.scalar_op_compiler.register_binary_op
 register_nary_op = scalar_compiler.scalar_op_compiler.register_nary_op
 register_ternary_op = scalar_compiler.scalar_op_compiler.register_ternary_op
 
@@ -133,6 +134,11 @@ def _(
     )
 
 
+@register_binary_op(ops.fillna_op)
+def _(left: TypedExpr, right: TypedExpr) -> sge.Expression:
+    return sge.Coalesce(this=left.expr, expressions=[right.expr])
+
+
 @register_nary_op(ops.case_when_op)
 def _(*cases_and_outputs: TypedExpr) -> sge.Expression:
     # Need to upcast BOOL to INT if any output is numeric

tests/unit/core/compile/sqlglot/expressions/snapshots/test_generic_ops/test_fillna/out.sql

@@ -0,0 +1,14 @@
+WITH `bfcte_0` AS (
+  SELECT
+    `int64_col` AS `bfcol_0`,
+    `float64_col` AS `bfcol_1`
+  FROM `bigframes-dev`.`sqlglot_test`.`scalar_types`
+), `bfcte_1` AS (
+  SELECT
+    *,
+    COALESCE(`bfcol_0`, `bfcol_1`) AS `bfcol_2`
+  FROM `bfcte_0`
+)
+SELECT
+  `bfcol_2` AS `int64_col`
+FROM `bfcte_1`

tests/unit/core/compile/sqlglot/expressions/test_generic_ops.py

@@ -225,6 +225,12 @@ def test_clip(scalar_types_df: bpd.DataFrame, snapshot):
     snapshot.assert_match(sql, "out.sql")
 
 
+def test_fillna(scalar_types_df: bpd.DataFrame, snapshot):
+    bf_df = scalar_types_df[["int64_col", "float64_col"]]
+    sql = utils._apply_binary_op(bf_df, ops.fillna_op, "int64_col", "float64_col")
+    snapshot.assert_match(sql, "out.sql")
+
+
 def test_hash(scalar_types_df: bpd.DataFrame, snapshot):
     col_name = "string_col"
     bf_df = scalar_types_df[[col_name]]