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Re: EXPLAIN in the slow query log
On 10/19/2013 7:26 PM, Eric Bergen wrote:
I don't understand why you need both the <explain> xml style tag and
the double # to denote an explain entry. I would prefer to see each
line prefixed with # explain: to make it similar to the other info
lines above and get rid of the <explain>.
Regardless the <explain> tags (it's a different story), it was my
suggestion to go with double ## rather than with '# explain' which
Sergei actually suggested himself. It seems to me that it's just as easy
to grep for '^##' as it is for '# explain:', while the variant with
double # is less excessive and more readable when you just look through
the log without pre-grepping.
I don't insist on the above though, if those who care prefer the prefix
with '# explain:' :)
By prefixing each line with # explain: it makes grepping explain plans
easier than looking for a start and end tag on different lines.
On Fri, Oct 18, 2013 at 4:59 PM, Sergei Golubchik <serg@xxxxxxxxxxx> wrote:
On Oct 18, Sergei Petrunia wrote:
After recent changes in the [SHOW] EXPLAIN architecture in MariaDB, it became
really easy to add an option to print EXPLAIN(*) output into the slow query log,
so I went ahead and implemented it.
@@log_slow_verbosity has got a 'explain' flag, and if it is on, the slow query
log entries look like this:
# Time: 131017 21:26:02
# User@Host: root[root] @ localhost 
# Thread_id: 2 Schema: dbt3sf1 QC_hit: No
# Query_time: 5.535819 Lock_time: 0.000350 Rows_sent: 1 Rows_examined: 65633
## id select_type table type possible_keys key key_len ref rows Extra
## 1 SIMPLE nation ref PRIMARY,n_name n_name 26 const 1 Using where; Using index
## 1 SIMPLE customer ref PRIMARY,i_c_nationkey i_c_nationkey 5 dbt3sf1.nation.n_nationkey 25124 Using index
## 1 SIMPLE orders ref i_o_custkey i_o_custkey 5 dbt3sf1.customer.c_custkey 7 Using index
select count(*) from customer, orders, nation where c_custkey=o_custkey and c_nationkey=n_nationkey and n_name='GERMANY';
It seems, pt-slow-query-log has no problem parsing this.
- Is this syntax OK?
Yes, assuming you use tabs as column separators, not spaces.
- Currently, some statements (like SHOW KEYS) print a dumb explain that
shows that full table scan is used. There is a suggestion to not print
EXPLAIN for any SHOW statements. Is this ok?
Yes. If users will complain we can always change it back later.
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