Analysis of MySQL Bug #89126: Table Definition Cache Crash and Configuration Recommendations

The article is part of the "Illustrated MySQL" series produced by the Aikesheng R&D team, aiming to deliver high‑quality technical content.

Bug description : MySQL Bug #89126 occurs when a database with many tables executes create table , leading to a crash due to exhaustion of the InnoDB table definition cache.

Phenomenon : Creating a large number of tables fills the table definition cache; subsequent create table ... statements have a certain probability of causing MySQL to crash, as shown in the stack trace image.

Principle diagram (viewable in landscape mode) illustrates the sequence:

Step 1 – create table ... inserts a new table definition into the cache and holds a reference, but the code releases the definition.

Step 2 – InnoDB performs cache reclamation and frees the already‑released table definition.

Step 3 – A subsequent create table ... uses the reference to the now‑freed definition, accessing reclaimed memory and causing a crash.

Fix – Change the behavior in step 1 to not release the table definition; this fix has not yet been accepted by the MySQL upstream.

Related knowledge points :

Table definition cache (table cache): InnoDB caches table definitions to reduce I/O pressure.

The cache maintains two data structures: a hash for fast lookup and an LRU (Least Recently Used) list for reclamation.

Cache reclamation runs periodically with two strategies – busy‑time and idle‑time – to minimize impact.

The cache can temporarily exceed its configured size, a design tied to the reclamation schedule.

The cache size limit is controlled by the table_definition_cache variable; the minimum is 400, and the default is calculated as min(400+table_open_cache/2,2000) .

Configuration suggestion : Unless you have tens of thousands of tables, set table_definition_cache slightly larger than the number of tables so that the cache can hold all definitions.

Further reading :

Bug description: https://bugs.mysql.com/bug.php?id=89126

MySQL official documentation on opening and releasing table definitions: https://dev.mysql.com/doc/refman/5.7/en/table-cache.html