How to Fix 'Lost connection to MySQL server during query' Errors When Processing Large Datasets?

How to Handle "Lost connection to MySQL server during query" Error

When working with extremely large datasets, it's common to encounter the error "Lost connection to MySQL server during query." This error occurs during the iteration of data rows, leading to connection loss and the obscuring of the last executed SQL query's position.

Improved Code Sample

To address this issue, the following code sample utilizes the SSCursor from the MySQLdb library, which supports automatic reconnection upon query failure:

import MySQLdb

class DB:
    conn = None

    def connect(self):
        self.conn = MySQLdb.connect(
            "hostname", "user", "*****", "some_table", cursorclass=MySQLdb.cursors.SSCursor
        )

    def query(self, sql):
        cursor = self.conn.cursor()
        cursor.execute(sql)
        return cursor

#
db = DB()
sql = "SELECT bla FROM foo"
data = db.query(sql)

for row in data:
    do_something(row)

Troubleshooting Tips

If the error persists, refer to the MySQL documentation on this specific error. Possible causes may include:

• Incorrect or oversized queries: Queries that are too large or contain errors can trigger connection closure. Increasing the server's max_allowed_packet variable and adjusting the client-side packet size might resolve this.
• Warnings: Starting MySQL with the --log-warnings=2 option generates a log file containing information about disconnected errors. This can provide further insight into the cause of the problem.

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