Explain query

Author: Adam Tal

Diff for: .gitignore

@@ -1,2 +1,4 @@
 .idea/*
 venv/*
+example.txt
+

Diff for: explain_query_locks.py

@@ -0,0 +1,123 @@
+import argparse
+
+import psycopg2
+from prettytable import PrettyTable
+
+LOCK_CHECK_QUERY = """
+    SELECT l.relation,
+           c.relname,
+           l.mode
+    FROM pg_locks l
+    JOIN pg_class c ON c.oid=l.relation
+    JOIN pg_stat_activity a on l.pid = a.pid
+    AND a.query NOT ILIKE '%pg_stat_activity%'
+    ORDER BY l.relation ASC;
+"""
+
+RELATION_ID = 'Relation ID'
+RELATION_NAME = 'Relation Name'
+LOCK_TYPE = 'Lock Type'
+
+
+def main(
+        user: str,
+        password: str,
+        host: str,
+        port: str,
+        database: str,
+        query: str,
+):
+    """Execute and rollback a query to see what locks it will take"""
+
+    # Create a DB connection that will "stage" but not commit the DB change
+    connection_for_schema_change = psycopg2.connect(
+        user=user,
+        password=password,
+        host=host,
+        port=port,
+        database=database,
+    )
+    connection_for_schema_change.autocommit = False
+    schema_change_cursor = connection_for_schema_change.cursor()
+
+    # Create a DB connection that will check what Locks are taken for "query"
+    connection_for_lock_check = psycopg2.connect(
+        user=user,
+        password=password,
+        host=host,
+        port=port,
+        database=database,
+    )
+    lock_check_cursor = connection_for_lock_check.cursor()
+
+    # Execute the query, but do not commit
+    schema_change_cursor.execute(query)
+
+    lock_check_cursor.execute(LOCK_CHECK_QUERY)
+
+    results = lock_check_cursor.fetchall()
+
+    connection_for_schema_change.rollback()
+
+    results_table = PrettyTable()
+
+    results_table.field_names = [RELATION_ID, RELATION_NAME, LOCK_TYPE]
+    results_table.align[RELATION_ID] = 'l'
+    results_table.align[RELATION_NAME] = 'l'
+    results_table.align[LOCK_TYPE] = 'l'
+
+    for relation_id, relation_name, lock_type in results:
+        results_table.add_row([relation_id, relation_name, lock_type])
+
+    print(results_table)
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+
+    parser.add_argument(
+        '--query',
+        required=True,
+        help='A DDL statement to explain',
+    )
+
+    parser.add_argument(
+        '--user',
+        required=True,
+        help='User for database connection',
+    )
+
+    parser.add_argument(
+        '--password',
+        required=True,
+        help='Password for database connection',
+    )
+
+    parser.add_argument(
+        '--host',
+        default='localhost',
+        help='Host for database connection',
+    )
+
+    parser.add_argument(
+        '--port',
+        default='5432',
+        help='Port for database connection',
+    )
+
+    parser.add_argument(
+        '--database',
+        required=True,
+        help='Database for database connection',
+    )
+
+    args = parser.parse_args()
+
+    main(
+        args.user,
+        args.password,
+        args.host,
+        args.port,
+        args.database,
+        args.query,
+    )

Diff for: requirements.txt

@@ -1 +1,2 @@
+prettytable==0.7.2
 psycopg2==2.8.4