To be honest, it is hardly worth having a dedicated method for
`where_sql`, since it is a reduced version of `to_sql`.
I've left it for now, but just made it more few code (used `to_sql`).
This partly reverts #35617.
#35617 includes unused code (for `InsertStatement`) and re-using query
annotation for `update_all` and `delete_all`, which has not been
discussed yet.
If a relation has any annotation, I think it is mostly for SELECT query,
so re-using annotation by default is not always desired behavior for me.
We should discuss about desired behavior before publishing the
implementation.
This patch has two main portions:
1. Add SQL comment support to Arel via Arel::Nodes::Comment.
2. Implement a Relation#annotate method on top of that.
== Adding SQL comment support
Adds a new Arel::Nodes::Comment node that represents an optional SQL
comment and teachers the relevant visitors how to handle it.
Comment nodes may be added to the basic CRUD statement nodes and set
through any of the four (Select|Insert|Update|Delete)Manager objects.
For example:
manager = Arel::UpdateManager.new
manager.table table
manager.comment("annotation")
manager.to_sql # UPDATE "users" /* annotation */
This new node type will be used by ActiveRecord::Relation to enable
query annotation via SQL comments.
== Implementing the Relation#annotate method
Implements `ActiveRecord::Relation#annotate`, which accepts a comment
string that will be appeneded to any queries generated by the relation.
Some examples:
relation = Post.where(id: 123).annotate("metadata string")
relation.first
# SELECT "posts".* FROM "posts" WHERE "posts"."id" = 123
# LIMIT 1 /* metadata string */
class Tag < ActiveRecord::Base
scope :foo_annotated, -> { annotate("foo") }
end
Tag.foo_annotated.annotate("bar").first
# SELECT "tags".* FROM "tags" LIMIT 1 /* foo */ /* bar */
Also wires up the plumbing so this works with `#update_all` and
`#delete_all` as well.
This feature is useful for instrumentation and general analysis of
queries generated at runtime.
We as Arm Treasure Data are using Optimizer Hints with a monkey patch
(https://gist.github.com/kamipo/4c8539f0ce4acf85075cf5a6b0d9712e),
especially in order to use `MAX_EXECUTION_TIME` (refer #31129).
Example:
```ruby
class Job < ApplicationRecord
default_scope { optimizer_hints("MAX_EXECUTION_TIME(50000) NO_INDEX_MERGE(jobs)") }
end
```
Optimizer Hints is supported not only for MySQL but also for most
databases (PostgreSQL on RDS, Oracle, SQL Server, etc), it is really
helpful to turn heavy queries for large scale applications.
Initially, `TOP` was introduced to support `limit` for MSSQL database.
Unlike PostgreSQL/MySQL/SQLite, MSSQL does not have native `LIMIT`/`OFFSET` support.
The commit adding `TOP` is 1a246f7161.
However, it figured out that `TOP` implementation was weak and it's not sufficient
to also support `OFFSET`, then `TOP` was substituted with
`ROW_NUMBER()` subquery in be48ed3071.
This is a well known trick in MSSQL -
https://stackoverflow.com/questions/2135418/equivalent-of-limit-and-offset-for-sql-server.
So now we don't need this `visit_Arel_Nodes_Top` at all.
It does nothing useful but also adds an extra space after `SELECT` when `LIMIT` is being
used for **any** database.
