CODEWITHSUNDEEP
rubyruby-on-rails-3ruby-on-rails-4activerecord
sonnyhe2002
sonnyhe2002

Reputation: 2121

Use Explain with Count

In Rails there is a useful method that can show how long a query takes and what query it is.

The method is 'explain'

So I can do:

User.where(name: 'Johnny').explain

And it will display the actual sql query.

the 'count' method also makes an sql query but since 'count' returns a number and not an active record, I cannot use explain on it. Is there any way to do that?

Upvotes: 1

Views: 295

Answers (1)

AnoE
AnoE

Reputation: 8345

This works:

User.select('count(*)').where(name: 'Johnny').explain

EDIT: This is indeed exactly the same:

irb(main):004:0> Benutzer.where(login: 'xxx').count
   (2.2ms)  SELECT COUNT(*) FROM "BENUTZER" WHERE "BENUTZER"."LOGIN" = 'xxx'
=> 0
irb(main):005:0> Benutzer.select("count(*)").where(login: 'xxx').explain
  Benutzer Load (0.9ms)  SELECT count(*) FROM "BENUTZER" WHERE "BENUTZER"."LOGIN" = 'xxx'
=> EXPLAIN for: SELECT count(*) FROM "BENUTZER"  WHERE "BENUTZER"."LOGIN" = 'xxx'
Plan hash value: 1339361075

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |     1 |    12 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |          |     1 |    12 |            |          |
|*  2 |   TABLE ACCESS FULL| BENUTZER |     1 |    12 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("BENUTZER"."LOGIN"='xxx')

Note
-----
   - dynamic sampling used for this statement (level=2)

Upvotes: 2

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