问题
See these two python code snippets,
conn = MySQLdb.connect(c['host'], c['user'], c['password'], c['db'])
cur = conn.cursor()
cur.execute("select * from geo_weathers;) -> **1147L**
and
conn = MySQLdb.connect(c['host'], c['user'], c['password'], c['db'], cursorclass=MySQLdb.cursors.SSCursor)
cur = conn.cursor()
cur.execute("select * from geo_weathers") -> **18446744073709551615L**
Why the returned number of rows are different in above two cases ? And just FYI there are 1147 rows in a table.
SSCursor is used for saving result at servers side. Is it the reason ? What all rows are affected by this select query ?
Does anyone know ?
回答1:
The standard cursor used by MySQLdb is a stored result cursor. This means that the entire result set is transferred from the server and cached in the client's memory as a result of the execute() call.
A SSCursor is a server side cursor will only return results to the client as they are requested.
The cursor execute() call will return the result of the MySql C API function mysql_affected_rows() which in turn returns the result of mysql_num_rows() for SELECT queries.
Because results are stored server side, the client does not know how many rows have been affected until it iterates over the results.
The documentation for mysql_num_rows() says this:
The use of mysql_num_rows() depends on whether you use mysql_store_result() or mysql_use_result() to return the result set. If you use mysql_store_result(), mysql_num_rows() may be called immediately. If you use mysql_use_result(), mysql_num_rows() does not return the correct value until all the rows in the result set have been retrieved.
If you want to obtain a count of the number of rows, use SELECT COUNT(*) from geo_weathers rather than rely on the result of the execute() call.
来源：https://stackoverflow.com/questions/14551009/mysqldb-cursors-cursor-execute-returns-different-values-in-case-of-different-cur