Why are almost all ROWS_EXAMINED values 0 in events_statements_* tables (performance_schema) !? - Source

Revision: 42185
User: Andras Fabian
Date: 2015-02-16 12:56

In the last days I started to explore the new possibilities of the perofmance_schema in 10.0.x. In this process I discovered the valuable event_statemens_* tables, which I could imagine using for some new monitoring approaches.

While studying the content of the event_statemens_* on our production DB server, I have noticed, that almost all ROWS_EXAMINED values were always equal 0! Only on one server, in the events_statements_summary_by_digest table could I find a few entries with ROWS_EXAMINED > 0 (but in those cases, both DIGEST and DIGEST_TEXT were NULL ???) .... which is a real pity, because this value seems to be the best one to be used as some kind of "load" indicator (and it can show very quickly, if a query needs an unreasonable amount of rows to be examined ... etc. ..).

Now the questions is, why is this happening? As far as I could see, all other values in the event_statemens_* tables seemed to have reasonable values. Only ROWS_EXAMINED was missing notoriously. Is there special reason for this? Is there some instrumentation setting needed to make this work? Is there some server variable which influences this? or is there some very special technical reason - from the servers logic - which prohibits to gather this value most of the time?

By the way, I have also observed - and this might coincide with this issue - that neither do I ever see EXAMINED_ROWS > 0  in `information_schema`.`PROCESSLIST`! Even if I fire a lung running query, which uses a table scan.

By the way, the only place where I always - and reliably - see ROWS_EXAMINED is in the slow_query_log!

I am really curious about the background (and possible reasons) for this - seemingly - strange behavior.

Thank you very much!

PS: we are - currently - running MariaDB 10.0.14 (on Ubuntu 12.04) in our production environment. But I could observe the same behavior on one - already updated - 10.0.16 dev machine.

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Why are almost all ROWS_EXAMINED values 0 in events_statements_* tables (performance_schema) !? - Source

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