Arel: Add support for FILTER clause (SQL:2003)

[Envek]

Reopened from rails/arel#518 due to the numerous requests of users (also in rails/arel#460).

Allows to write following Ruby code:

Model.all.pluck(
  Arel.star.count.as('records_total').to_sql,
  Arel.star.count.filter(Model.arel_table[:some_column].not_eq(nil)).as('records_filtered').to_sql,
)

to get following SQL:

SELECT
  COUNT(*) AS records_total  
  COUNT(*) FILTER (WHERE some_column IS NOT NULL) AS records_filtered
FROM models

Database support:

1. PostgreSQL 9.4+ (december 2014, release notes)
2. SQLite 3.30+ (october 2019, release notes)

See:

• http://modern-sql.com/feature/filter
• https://www.postgresql.org/docs/9.4/static/sql-expressions.html#SYNTAX-AGGREGATES
• https://sqlite.org/lang_aggfunc.html#aggfilter

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[Envek]

Oh bot, just look at these reactions under first comment! How you even could think about closing this pull request? Heartless piece of code!

[Envek]

@sunny thanks for catching this weird naming of test examples (most probably I copied them from some project, but it was long time ago).

Applied your suggestions and rebased on top of fresh main branch.

[simi]

I really like this extension of Arel. As a followup we can open another case to expose it via ActiveRecord public API. Also it is not supported by MySQL, but it can be implemented using CASE (https://modern-sql.com/feature/filter#conforming-alternatives). It could be added to MySQL adapter later as well if needed.

[benedikt]

I'd love to see this merged!