Fix(bigquery): always infer concat type as either bytes or string (#5085)

Author: georgesittas

sqlglot/dialects/bigquery.py

@@ -353,6 +353,17 @@ def _json_extract_sql(self: BigQuery.Generator, expression: JSON_EXTRACT_TYPE) -
     return sql
 
 
+def _annotate_concat(self: TypeAnnotator, expression: exp.Concat) -> exp.Concat:
+    annotated = self._annotate_by_args(expression, "expressions")
+
+    # Args must be BYTES or types that can be cast to STRING, return type is either BYTES or STRING
+    # https://cloud.google.com/bigquery/docs/reference/standard-sql/string_functions#concat
+    if not annotated.is_type(exp.DataType.Type.BINARY, exp.DataType.Type.UNKNOWN):
+        annotated.type = exp.DataType.Type.VARCHAR
+
+    return annotated
+
+
 class BigQuery(Dialect):
     WEEK_OFFSET = -1
     UNNEST_COLUMN_ONLY = True
@@ -433,7 +444,7 @@ class BigQuery(Dialect):
                 exp.Substring,
             )
         },
-        exp.Concat: lambda self, e: self._annotate_by_args(e, "expressions"),
+        exp.Concat: _annotate_concat,
         exp.Sign: lambda self, e: self._annotate_by_args(e, "this"),
         exp.Split: lambda self, e: self._annotate_by_args(e, "this", array=True),
     }

tests/fixtures/optimizer/annotate_functions.sql

@@ -205,6 +205,14 @@ STRING;
 CONCAT(tbl.bin_col, tbl.bin_col);
 BINARY;
 
+# dialect: bigquery
+CONCAT(0, tbl.str_col);
+STRING;
+
+# dialect: bigquery
+CONCAT(tbl.str_col, 0);
+STRING;
+
 # dialect: bigquery
 LEFT(tbl.str_col, 1);
 STRING;