Why Do Repeated MySQL Queries in Python Return Identical Results?

Persistent Data Retrieval from Repeated MySQL Queries in Python

In Python scripting, it's essential to frequently query MySQL databases to capture dynamic data. When relying on loops to retrieve such data, it can be confusing to find that subsequent fetches return identical results.

Issue

In the given Python script, a loop is used to repeatedly execute a MySQL query and store the results in a list:

<code class="python">for i in range(listSize):
    # ...
    # SQL query
    sql = "SELECT * FROM table"
    # ...</code>

Despite expectations that the loop would refresh the data retrieved from the database at each iteration, the script retrieves the same results repeatedly.

Solution

The crux of the issue lies in MySQL's transaction isolation levels. By default, InnoDB, the storage engine used by MySQL, operates under READ COMMITTED isolation level, which ensures the following:

"Consistent reads within the same transaction read the snapshot established by the first read."

This means that subsequent queries within a transaction retrieve the data as it existed when the transaction was initiated. To address this, each iteration of the loop must end with a mydb.commit() command.

<code class="python">import ...
# ...
while True:
    # ...
    # SQL query
    sql = "SELECT * FROM table"
    # ...
    mydb.commit()</code>

By committing the connection, the script forces the database to capture changes made during the transaction, ensuring that subsequent queries reflect the current state of the database.

Explanation

Each loop iteration within a MySQL connection is treated as a separate transaction, which by default reads from a consistent snapshot. Committing the connection at the end of each iteration ensures that the subsequent transaction reads the most recent snapshot, reflecting the changes made in the previous transaction.

The above is the detailed content of Why Do Repeated MySQL Queries in Python Return Identical Results?. For more information, please follow other related articles on the PHP Chinese website!