The EXPLAIN FOR CONNECTION statement was executed against a thread running a command that MySQL cannot explain, triggering ER_EXPLAIN_NOT_SUPPORTED (code 3012, SQLSTATE HY000).
MySQL ER_EXPLAIN_NOT_SUPPORTED (3012) occurs when you run EXPLAIN FOR CONNECTION on a thread that is not executing SELECT, UPDATE, INSERT, DELETE, or REPLACE. Switch the target connection to a supported DML statement or use PERFORMANCE_SCHEMA views to inspect the query.
ER_EXPLAIN_NOT_SUPPORTED
MySQL raises ER_EXPLAIN_NOT_SUPPORTED when you issue EXPLAIN FOR CONNECTION against a running thread that is not executing a Data Manipulation Language statement. The server can only generate an execution plan for SELECT, UPDATE, INSERT, DELETE, or REPLACE. If the target thread is idle or running a different command, the explain request fails.
This error first appeared in MySQL 5.7.2 alongside the new EXPLAIN FOR CONNECTION feature. Understanding why it fires is key to debugging long-running queries without interrupting production traffic.
The primary trigger is using EXPLAIN FOR CONNECTION on a thread that is performing an unsupported operation such as CREATE TABLE, ALTER TABLE, LOAD DATA, or a metadata query. MySQL rejects the request because it cannot build an execution plan for those commands.
The error also appears if you target a sleeping connection, a thread in the “statistics” state, or a connection already finished. Even when the original statement was a SELECT, once it completes the explain operation is no longer possible.
First verify the target thread in information_schema.processlist or performance_schema. Ensure COMMAND is one of the supported DML types. If not, wait until the thread runs a supported query or choose another thread.
When you must analyze unsupported operations, rely on performance_schema events_statements_history or the slow query log instead. These sources show statement text and timing without needing EXPLAIN FOR CONNECTION.
During migrations, DBAs sometimes run EXPLAIN FOR CONNECTION on ALTER statements to gauge progress, which triggers the error. Use SHOW PROCESSLIST for progress and ALTER TABLE ... ALGORITHM=INPLACE where possible.
Developers profiling idle connections often hit the error. Filter sleeping threads from your process list query or enable wait_timeout to close them automatically.
Always inspect processlist or performance_schema threads before calling EXPLAIN FOR CONNECTION. Automate a check that COMMAND is among SELECT, UPDATE, INSERT, DELETE, or REPLACE.
Capture execution plans proactively with EXPLAIN FORMAT=JSON during development. Galaxy’s AI copilot can insert these statements automatically so you rarely need live connection explains in production.
ER_NO_SUCH_THREAD (thread id X) appears when the target connection no longer exists. Query processlist first to confirm validity.
ER_TOO_MANY_CONCURRENT_TRXS arises when many concurrent EXPLAIN FOR CONNECTION calls overload metadata locks. Limit the number of concurrent explain attempts or stagger diagnostics.
The thread is executing DDL, administrative, or metadata statements that cannot be explained.
The chosen connection finished its work, so no active statement exists to explain.
An automation script may pass the wrong thread id or reuse stale ids, leading to the error.
The specified connection id does not exist or has already terminated.
The server has reached the maximum number of connections; diagnostics may fail.
Too many concurrent transactions can block explain attempts by holding metadata locks.
No. The feature only supports SELECT, UPDATE, INSERT, DELETE, and REPLACE. Use SHOW CREATE TABLE or performance_schema for DDL insights.
If the SELECT finished before you issued EXPLAIN FOR CONNECTION, MySQL cannot retrieve the plan and raises ER_EXPLAIN_NOT_SUPPORTED.
Monitor progress with SHOW PROCESSLIST and information_schema.innodb_trx. For execution plan details, evaluate the statement on a staging database using EXPLAIN FORMAT=JSON.
Galaxy surfaces real-time process lists and flags unsupported threads, guiding you to valid connection ids. Its AI copilot also injects EXPLAIN FOR CONNECTION only when the target command is supported.
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