Error 1151 signals that the server has reached the limit for INSERT DELAYED handler threads, blocking new delayed operations until threads free up.
MySQL Error 1151: ER_UNUSED2 (Too many delayed threads in use) occurs when the INSERT DELAYED thread pool hits its limit. Reduce concurrent delayed inserts or raise the delayed_insert_threads setting to resolve the issue.
Too many delayed threads in use
MySQL raises Error 1151 with SQLSTATE HY000 when it cannot spawn another thread to handle an INSERT DELAYED statement because all delayed insert threads are busy.
The server refuses additional delayed inserts to protect stability. Normal INSERT statements continue to work, but applications relying on delayed inserts stall or fail.
The message surfaces during execution of INSERT DELAYED or LOAD DATA INFILE ...
DELAYED statements on MySQL 5.x and early 8.x builds where delayed inserts are still supported.
It is most common on high-throughput workloads that queue thousands of delayed rows per second, exhausting the limited thread pool.
Unresolved, the error causes data ingress delays, application timeouts, and potential data loss if retry logic is absent. Eliminating the bottleneck restores ingest speed and system reliability.
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INSERT DELAYED was removed in MySQL 8.0.20. Upgrade paths should migrate to normal INSERT or bulk-load methods.
Yes. Set delayed_insert_limit=1 and delayed_queue_size=0, then avoid using the DELAYED keyword in client code.
Increasing the value helps until CPU or I/O become bottlenecks. Monitor system load to balance resources.
Galaxy flags INSERT DELAYED usage in scripts, suggests modern patterns, and lets teams refactor collaboratively, preventing thread exhaustion errors.
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