How to delete a table rows and keep top X rows

Question

I have this large table in MySQL incident_archive that has millions of records, I want to sort the rows by created column and keep the Top X rows and delete the rest, what is the most efficient way to do this.

So far I came up with this solution in Python:

def do_delete_archive(rowsToKeep):

    if rowsToKeep > 0:
        db_name = find_environment_value('DB_NAME', False, "dbname")
        db_host = find_environment_value('DB_HOST', False, "host")
        db_user = find_environment_value('DB_USER', False, "username")
        db_pass = find_environment_value('DB_PASS', False, "password")
        db = MySQLdb.connect(host=db_host,user=db_user,passwd=db_pass,db=db_name)
        cursor = db.cursor()            
        
        sql = f"""DELETE FROM `incident_archive` 
                    WHERE incident_id NOT IN 
                    ( SELECT incident_id FROM 
                    ( SELECT incident_id FROM `incident_archive` ORDER BY created DESC LIMIT {rowsToKeep}) foo) LIMIT 10000;"""
        
        try:
            rowcount = rowsToKeep+ 1
            while rowcount > rowsToKeep:
                cursor.execute(sql)
                db.commit()
                rowcount = cursor.rowcount
                print(f"--- Affected Rows: {rowcount} ---")

        except:
            db.rollback()

The issue that I have here if rowsToKeep has value more or equal 10000 this approach will not work, what is a better way to this process?

**Note: rowsToKeep value is dynamic, meaning it can change.

Asked By: DeadlyDagger

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Source

Answer 1

I came up with solution below:

NOTE: threshold is the variable contains number of max records we want to keep 1000 in my example

sqlCreate = f"""CREATE TABLE new_incident_archive LIKE incident_archive;"""
print(f"Running query is: {sqlCreate}")
cursor.execute(sqlCreate)
print(f"Done with: {sqlCreate}")

sqlInsert = f"""INSERT INTO new_incident_archive SELECT * FROM `incident_archive` ORDER BY created DESC LIMIT {threshold}"""
print(f"Running query is: {sqlInsert}")
cursor.execute(sqlInsert)
db.commit()
print(f"Done with: {sqlInsert}")

sqlDrop = f"""DROP TABLE incident_archive"""
print(f"Running query is: {sqlDrop}")
cursor.execute(sqlDrop)
print(f"Done with: {sqlDrop}")

sqlRename = f"""RENAME TABLE `new_incident_archive` TO `incident_archive`;"""
print(f"Running query is: {sqlRename}")
cursor.execute(sqlRename)
print(f"Done with: {sqlRename}")

Answered By: DeadlyDagger

How to delete a table rows and keep top X rows

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