Proposal: add support for EXPLAIN statement

[leandrocp]

During development a very common process is to optimize complex queries by using the EXPLAIN sql statement, which as for now requires to play with to_sql, copy & paste, manually building the query and calling it on a SQL client. This PR proposes to add Repo.explain/2 function to ease this process and improve the developer experience.

Of course It's just a WIP to gather feedback of the core team, as it's missing:

• Maybe improve the function return? current output is just a string like "Seq Scan on barebones b0 (cost=0.00..30.40 rows=2040 width=4)"
• Call EXPLAIN inside a transaction to avoid side effects
• Support other statements besides SELECT
• Pass options to the EXPLAIN statement
• Implement callback for all adapters
• Improve tests

ecto-sql implementation: elixir-ecto/ecto_sql#231

Please note I'm not an expert in Ecto internals so it may not be the best possible implementation.

[josevalim]

Hi @leandrocp! Thanks so much for the PR. I think however we can implement this directly in Ecto.SQL. We probably don't need to change the planner and instead we can prepare it using :all and then append the EXPLAIN instructions. WDYT?

[leandrocp]

The only reason I changed the planner was because I wasn't sure the best way to implement this and it looked right :)
So I'm fine with implementing this on ecto_sql side, maybe similar to to_sql?

[josevalim]

So I'm fine with implementing this on ecto_sql side, maybe similar to to_sql?

Yes, exactly!

[josevalim]

I will go ahead and close this but ping me if you feel like this is necessary. :)