Fix double EXPLAIN when calling explain on queryset (#654)

* Fix double EXPLAIN when calling explain on queryset

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Albert Wang <git@albertyw.com>

Author: 3 people

project/tests/test_execute_sql.py

@@ -32,7 +32,8 @@ def mock_sql(mock_query_params):
             spec_set=['supports_explaining_query_execution'],
             supports_explaining_query_execution=True
         ),
-        ops=NonCallableMock(spec_set=['explain_query_prefix']),
+        ops=NonCallableMock(spec_set=['explain_query_prefix'],
+                            explain_query_prefix=Mock(return_value='')),
     )
 
     return mock_sql_query, mock_query_params
@@ -131,6 +132,7 @@ def test_explain_simple(self):
         mock_cursor = sql.connection.cursor.return_value.__enter__.return_value
         sql.connection.ops.explain_query_prefix.return_value = prefix
         execute_sql(sql)
+        self.assertNotIn(prefix, params)
         params = tuple(force_str(param) for param in params)
         mock_cursor.execute.assert_called_once_with(f"{prefix} {_simple_mock_query_sql}", params)
 
@@ -141,6 +143,7 @@ def test_explain_unicode(self):
         mock_cursor = sql.connection.cursor.return_value.__enter__.return_value
         sql.connection.ops.explain_query_prefix.return_value = prefix
         execute_sql(sql)
+        self.assertNotIn(prefix, params)
         params = tuple(force_str(param) for param in params)
         mock_cursor.execute.assert_called_once_with(f"{prefix} {_simple_mock_query_sql}", params)
 
@@ -151,4 +154,5 @@ def test_explain_non_unicode(self):
         mock_cursor = sql.connection.cursor.return_value.__enter__.return_value
         sql.connection.ops.explain_query_prefix.return_value = prefix
         execute_sql(sql)
+        self.assertNotIn(prefix, params)
         self.assertFalse(mock_cursor.execute.called)

silk/sql.py

@@ -53,7 +53,11 @@ def _explain_query(connection, q, params):
 
         # currently we cannot use explain() method
         # for queries other than `select`
-        prefixed_query = f"{prefix} {q}"
+        if q.upper().startswith(prefix.upper()):
+            # to avoid "EXPLAIN EXPLAIN", do not add prefix
+            prefixed_query = q
+        else:
+            prefixed_query = f"{prefix} {q}"
         with connection.cursor() as cur:
             try:
                 params_str = tuple(force_str(param) for param in params)