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What's new in mk-query-digest: EXPLAIN sparklines

This month’s Maatkit release has some nice new features and improvements to mk-query-digest. There is one that deserves its own blog post: EXPLAIN sparklines.

A “sparkline” is a simple type of chart that conveys important information without the details. We added a kind of ASCII sparkline to mk-query-digest to convey important information about the query’s EXPLAIN plan so you can see if the query is “bad” or not. It is kind of a cryptic geek code that you will need some help decoding. It’s intentionally compact, so that it can fit in the “profile” that mk-query-digest prints out from a normal report.

Here is an example of the profile report:

# Profile
# Rank Query ID           Response time    Calls R/Call   Apdx V/M   EXPLAIN Item
# ==== ================== ================ ===== ======== ==== ===== ======= =====
#    1 0x808CDA06B6EB3D5A     0.0141 83.5%     2   0.0071 1.00  0.00 aa      SELECT test.t
#    2 0x8305A7D4195D2096     0.0011  6.7%     6   0.0002 1.00  0.00 aa      SELECT test.t

The EXPLAIN column appears if you add the –explain option to mk-query-digest. Note: while writing this post I discovered a bug in the new functionality, which is now fixed in trunk, so if you want to use this you’ll need to ‘wget maatkit.org/trunk/mk-query-digest’ to get a version that doesn’t have the bug.

In this case, both queries are shown as aa. What is that? It’s our geek code, one character per table in the EXPLAIN plan. And if we look at the documentation, ‘a’ is the shorthand for Type=ALL:

The abbreviated table access codes are:

  a  ALL
  c  const
  e  eq_ref
  f  fulltext
  i  index
  m  index_merge
  n  range
  o  ref_or_null
  r  ref
  s  system
  u  unique_subquery

So “aa” is shorthand for “table scan the first table, and do a cross-join with the second table by scanning it too.” That’s a terrible query plan! Someone needs to fix their SQL or add some indexes or something.

The code includes a couple of other small but important bits of data about the EXPLAIN plan:

  • If the letter is upper-case, it means there was a “Using index” in the Extra column for that table, so it’s accessed through a covering index.
  • If there was a temporary table or filesort, it appears as T or F in the output, separated by a “>” character. This can appear before or after the rest of the EXPLAIN, depending on the method MySQL uses to order the results.

Here are some more examples so you can practice reading the results:

  • TF>cRn is a three-table join: the first table is treated as a constant, the next table is accessed by ‘ref’ with a covering index, and the final table is accessed by an index range scan. There is a temp table and filesort on the first or second table. (We actually know that it’s the second table, because the first table is treated as a constant.)
  • aeeeE is something like a star-schema join in a data warehousing query. The first table is accessed via a full table scan. It’s probably the fact table. The second, third, and fourth tables are accessed through an eq_ref method; they are probably dimension tables. The last table is also an eq_ref, but it uses a covering index.

Posted on Fri, Dec 17, 2010. Approximately 600 Words.

Databases