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POChronic stale results using MySQLdb in Python
 

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  1. POChronic stale results using MySQLdb in Python
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    <p>My Python program queries a set of tables in a MySQL DB, sleeps for 30 seconds, then queries them again, etc. The tables in question are continuously updated by a third-party, and (obviously) I would like to see the new results every 30 seconds.</p> <p>Let's say my query looks like this:</p> <pre><code>"select * from A where A.key &gt; %d" % maxValueOfKeyFromLastQuery </code></pre> <p>Regularly I will see that my program stops finding new results after one or two iterations, even though new rows <em>are</em> present in the tables. I know new rows are present in the tables because I can see them when I issue the identical query from interactive mysql (i.e. not from Python).</p> <p>I found that the problem goes away in Python if I terminate my connection to the database after each query and then establish a new one for the next query.</p> <p>I thought maybe this could be a server-side caching issue as discussed here: <a href="https://stackoverflow.com/questions/2474609/explicit-disable-mysql-query-cache-in-some-parts-of-program">Explicit disable MySQL query cache in some parts of program</a></p> <p>However:</p> <ol> <li><p>When I check the interactive mysql shell, it says that caching is on. (So if this is a caching problem, how come the interactive shell doesn't suffer from it?)</p></li> <li><p>If I explicitly execute <code>SET SESSION query_cache_type = OFF</code> from within my Python program, the problem still occurs.</p></li> </ol> <p>Creating a new DB connection for each query is the only way I've been able to make the problem go away.</p> <p>How can I get my queries from Python to see the new results that I know are there?</p>
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    1. COYou should not use the `%` format operator in queries. Use `%s` for all placeholders (no matter what their type is) and pass a tuple/list with the values so they can be sanitized properly.
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    2. COJust a guess: Make sure that your queries that read from the db are not isolated from other changes by a transaction. I.e. create a new transaction before each read.
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    3. CO@ThiefMaster: `%d` is actually okay in queries, because it forces the substitution to be an integer, which cannot be used for SQL injection attacks.
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