Handle decimal types in query results

Since #6687, we don't serialize query results as JSON
before returning them. This is fine, except for the
query results data source which needs to pass the
data directly to sqlite3, and doesn't know how to
do that with the decimal types that are occasionally
returned by (at least) the PostgreSQL query runner:

https://www.psycopg.org/docs/faq.html#problems-with-type-conversions

redash/query_runner/query_results.py

@@ -1,3 +1,4 @@
+import decimal
 import hashlib
 import logging
 import re
@@ -105,6 +106,8 @@ def fix_column_name(name):
 def flatten(value):
     if isinstance(value, (list, dict)):
         return json_dumps(value)
+    elif isinstance(value, decimal.Decimal):
+        return float(value)
     else:
         return value
 

tests/query_runner/test_query_results.py

@@ -1,3 +1,4 @@
+import decimal
 import sqlite3
 from unittest import TestCase
 
@@ -107,6 +108,16 @@ def test_creates_table_with_non_ascii_in_column_name(self):
         create_table(connection, table_name, results)
         connection.execute("SELECT 1 FROM query_123")
 
+    def test_creates_table_with_decimal_in_column_value(self):
+        connection = sqlite3.connect(":memory:")
+        results = {
+            "columns": [{"name": "test1"}, {"name": "test2"}],
+            "rows": [{"test1": 1, "test2": decimal.Decimal(2)}],
+        }
+        table_name = "query_123"
+        create_table(connection, table_name, results)
+        connection.execute("SELECT 1 FROM query_123")
+
     def test_shows_meaningful_error_on_failure_to_create_table(self):
         connection = sqlite3.connect(":memory:")
         results = {"columns": [], "rows": []}