log_slow_filter=qc_miss inconsistent with QC_hit: in the slow query log

[Laurynas Biveinis]

We cannot fix this for 5.1 and 5.5, but we should consider replacing the qc_miss option value with qc_hit for 5.6.

[Raghavendra D Prabhu (raghavendra-prabhu)]

Actually, both qc_hit and qc_miss will be better.

However, currently qc_miss can be filtered, so documentation for 5.5 http://www.percona.com/doc/percona-server/5.5/diagnostics/slow_extended_55.html -- needs to be fixed (s/qc_hit/qc_miss), documentation for 5.1 PS is correct.

The current logic for qc_miss is as follows:

1. ((thd->variables.log_slow_filter & SLOG_F_QC_NO) &&
        (thd->query_plan_flags & QPLAN_QC))))

2.
        "# QC_Hit: %s Full_scan: %s Full_join: %s Tmp_table: %s Tmp_table_on_disk: %s\n" \
        "# Filesort: %s Filesort_on_disk: %s Merge_passes: %lu\n",
        ((thd->query_plan_flags & QPLAN_QC) ? "Yes" : "No"),

So, from #1 and #2,

        if filter has query cache miss set and query was cached, it skips logging it, and QC_Hit will 1 but will not be logged

        if filter has query cache miss set and query indeed missed the cache,it logs it and QC_Hit will be logged as 0 in the log

Now, the rationale behind logging query misses with a filter may have been that query cache hits (qc_hits with QC_Hit 1) generally aren't slow whereas qc misses generally hurt the performance.

Also, people may be interested in slow queries which don't fit the profile of filesort*, full_*, tmp_* but are still interested in ones which escape that, setting log_slow_filter=qc_miss seems to be the only way to log such queries.

However, people may also be interested in having query cache hits specifically filtered (to diagnose query cache issues), so having that makes sense as well. I guess, having something like invertible elements is better (not sure if such a type is supported in 5.6) like -- log_slow_filter=!qc_miss,!filesort

[Alexey Kopytov (akopytov)]

OK, so we don't actually have to fix anything in 5.6, the naming is consistent. Let's keep this bug as a feature request to introduce qc_hit in addition to qc_miss. The "!qc_miss,!filesort" syntax is not supported though.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2380