How to Solve MySQL\'s \'Can\'t Reopen Table\' Error When Self-Joining?

Overcoming MySQL's "Can't Reopen Table" Error

When working with MySQL, you may encounter the frustrating "Can't reopen table" error when attempting to reuse a temporary table multiple times. This limitation can hinder scalability and performance in certain scenarios.

To understand the issue, consider the following query:

SELECT
    *
FROM search
INNER JOIN search f1 ON f1.baseID = search.baseID AND f1.condition = condition1
INNER JOIN search f2 ON f2.baseID = search.baseID AND f2.condition = condition2
...
INNER JOIN search fN ON fN.baseID = search.baseID AND fN.condition = conditionN

In this example, the "search" table is joined onto itself multiple times using temporary tables to filter specific rows. However, MySQL prohibits reopening temporary tables.

A Simple Workaround: Duplicating the Temporary Table

A straightforward solution is to simply duplicate the temporary table before using it in each join. This ensures that each join has its own unique instance of the temporary table, eliminating the "Can't reopen table" error.

This approach works well if the temporary table is relatively small, as is often the case with temporary tables. However, for larger temporary tables, duplicating the table may not be practical.

Exploration and Future Solutions

As mentioned by Kris in the original question, using the GROUP_CONCAT function is not suitable in this specific scenario. However, it remains a viable solution for certain use cases.

The "Can't reopen table" limitation is a known issue in MySQL. While there are workarounds and alternative approaches, a true solution remains elusive. It would be beneficial for future MySQL releases to address this limitation and enhance the flexibility of working with temporary tables.

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