indexing - understanding perf of mysql query using explain extended -


i trying understand performance of sql query using mysql. indexes on pk, query failed complete in on 10mins. have added indexes on columns used in clauses (timestamp, hostname, path, type) , query completes in approx 50seconds -- still seems long time not seem overly complex query.

so, i'd understand query causing this. assumption inner subquery in someway causing explosion in number of comparisons necessary.

there 2 tables involved:

storage (~5,000 rows / 4.6mb ) , machines (12 rows, <4k)

the query follows:

select t.hostname, t.path, t.used_pct,        t.used_gb, t.avail_gb, t.timestamp, machines.type type       storage t       join machines on t.hostname = machines.hostname       timestamp = ( select max(timestamp) storage st                             st.hostname = t.hostname ,                                               st.path = t.path)       , (machines.type = 'nfs')       order used_pct desc

an explain extended query returns following:

id       select_type        table     type       possible_keys        key          key_len    ref                            rows     filtered      1        primary            machines  ref        hostname,type        type         768        const                          1        100.00        using where; using temporary; using filesort 1        primary            t         ref        fk_hostname          fk_hostname  768        monitoring.machines.hostname   4535     100.00        using 2        dependent subquery st        ref        fk_hostname,path     path         1002       monitoring.t.path              648      100.00        using

noticing 'extra' column row 1 includes 'using filesort' , question: mysql explain query understanding states "using filesort sorting algorithm mysql isn't able use index sorting , therefore can't complete sort in memory."

what nature of query causing slow performance?

why necessary mysql use 'filesort' query?

indexes don't populated, there create them. that's why inserts , updates become slower more indexes have on table.

your query runs fast after first time because whole result of query put cache. see how fast query without using cache can 

select sql_no_cache t.hostname ...

mysql uses filesort order by or in case determine maximum value timestamp. instead of going through possible values , memorizing value greatest, mysql sorts values descending , picks first one.

so, why query slow? 2 things jumped eye.

1) subquery 

  timestamp = ( select max(timestamp) storage st                         st.hostname = t.hostname ,                                           st.path = t.path)

gets evaluated every (hostname, path). have try index on timestamp (btw, discourage naming columns keywords / datatypes). if alone doesn't help, try rewrite query. there 2 excellent examples in mysql manual: the rows holding group-wise maximum of column.

2) minor issue, seems joining on char/varchar fields. numbers / ids faster.


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