What's new in Edge Rails: EXPLAIN:
There are some new features related to EXPLAIN in the forthcoming Ruby on Rails 3.2 we'd like
to share:
There are some new features related to EXPLAIN in the forthcoming Ruby on Rails 3.2 we'd like
to share:
- Running EXPLAIN manually
- Automatic EXPLAIN for slow queries
- Silencing automatic EXPLAIN
sqlite3, mysql2, and postgresql.Running EXPLAIN Manually
You can now run EXPLAIN on the SQL generated by a relation this way:
User.where(:id => 1).joins(:posts).explain
Please note that
explain runs the query or queries and asks the database for their respective query plan afterwards. This is because due to eager loading a relation may trigger several queries to fetch the records and their associations, and in such cases some queries need the result of the previous ones.
If the relation triggers several queries the method still returns a single string with all the query plans. This is an output meant for human consumption so we preferred to present everything as a string in a format which is familiar right away rather than a structure.
Automatic EXPLAIN For Slow Queries
Rails 3.2 has the ability to help in detecting slow queries.
New applications get
config.active_record.auto_explain_threshold_in_seconds = 0.5
in config/environments/development.rb. Active Record monitors queries and if they take more than that threshold their query plan will be logged using
warn.
That works for anything running
find_by_sql (which is almost everything, since most of Active Record ends up calling that method). In the particular case of relations, the threshold is compared against the total time needed to fetch the records, not against the time taken by each individual query. Because conceptually we are concerned with the cost of the callUser.where(:id => 1).joins(:posts).explain
rather than the cost of the different queries that call may trigger due to the implementation.
By default the threshold is
nil in the test and production environments, which means the feature is disabled.
The value of that parameter is
nil also if the threshold is not set, so existing applications will need to add it by hand if they migrate to 3.2 to be able to enable automatic EXPLAIN.Silencing Automatic EXPLAIN
With automatic EXPLAIN enabled, it could still be the case that some queries are just slow and you know they have to be. For example, a heavyweight report in the backoffice.
The macro
silence_auto_explain allows you to avoid having EXPLAIN run repeatedly in those areas of code:ActiveRecord::Base.silence_auto_explain do
# no automatic EXPLAIN here
endInterpreting Query Plans
The interpretation of the query plans is another topic, these are some pointers:- SQLite: EXPLAIN QUERY PLAN
- MySQL: EXPLAIN Output Format
- PostgreSQL: Using EXPLAIN
