Log the plan of the currently running query (v54)#78
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Currently, we have to wait for the query execution to finish to know its plan either using EXPLAIN ANALYZE or auto_explain. This is not so convenient, for example when investigating long-running queries on production environments. To improve this situation, this patch adds pg_log_query_plan() function that requests to log the plan of the currently executing query. On receipt of the request, ExecProcNodes of the current plan node and its subsidiary nodes are wrapped with ExecProcNodeFirst, which implelements logging query plan. When executor executes the one of the wrapped nodes, the query plan is logged. Our initial idea was to send a signal to the target backend process, which invokes EXPLAIN logic at the next CHECK_FOR_INTERRUPTS() call. However, we realized during prototyping that EXPLAIN is complex and may not be safely executed at arbitrary interrupt points. By default, only superusers are allowed to request to log the plans because allowing any users to issue this request at an unbounded rate would cause lots of log messages and which can lead to denial of service. Todo: Consider removing the PID from the log output of pg_log_backend_memory_contexts() and pg_log_query_plan(). For detail, see the discussion: https://www.postgresql.org/message-id/CA%2BTgmoY81r7npTS34N_5MLA_u6ghfor5HoSaar53veXUYu1OxQ%40mail.gmail.com
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