I've experienced this issue in our app, some hints only works on Top
level query (e.g. `MAX_EXECUTION_TIME`).

When using `select` with `'DISTINCT( ... )'` if you use method `size` on a non loaded relation it overrides the column selected by passing `:all` so it returns different value than count.

This fixes #35214

Currently custom attributes are always qualified by the table name in
the generated SQL wrongly even if the table doesn't have the named
column, it would cause an invalid SQL error.

Custom attributes should only be qualified if the table has the same
named column.

Since Ruby 2.6.0 NilClass#to_d is returning `BigDecimal` 0.0, this
breaks `average` compatibility with prior Ruby versions. This patch
makes `average` return `nil` in all Ruby versions when there are no
rows.

This reverts 27c6c071 since `arel_attr.to_s` is not right way to avoid
the type error.

That to_s returns `"#<struct Arel::Attributes::Attribute ...>"`, there
is no reason to match the regex to the inspect form.

And also, the regex path is not covered by our test cases. I've tweaked
the regex for redundant part and added assertions for the regex path.

In cases where the MatchData object is not used, this provides a speed-up:
https://github.com/JuanitoFatas/fast-ruby/#stringmatch-vs-stringmatch-vs-stringstart_withstringend_with-code-start-code-end

* Add Relation#pick as short-hand for single-value plucks

With #31615 `type_for_attribute` accepts either
a symbol as well as a string. `has_attribute?` and `attribute_alias`
also accept either. Since these methods call `to_s` on the argument,
we no longer need to do that at the call site.

This is a regression caused by 6beb4de7.

In PostgreSQL, ORDER BY expressions must appear in SELECT list when
using DISTINCT.

When using `count(:all)` with eager loading, Active Record enforces
DISTINCT to count the driving table records only. 6beb4de7 was caused the
regression because `count(:all)` with DISTINCT path no longer removes
ORDER BY.

We need to ignore ORDER BY when DISTINCT is enforced, otherwise not
always generated valid SQL for PostgreSQL.

Fixes #31783.

Currently `count(:all)` with `distinct` doesn't work correctly because
SELECT list is always replaced to `*` or primary key in that case even
if having custom SELECT list.

And also, PostgreSQL has a limitation that ORDER BY expressions must
appear in select list for SELECT DISTINCT.

Therefore, we should not replace custom SELECT list when using
`count(:all)` with `distinct`.

Closes #31277.

Fixes #30315.

This is the fix for the regression of #29848.

In #29848, I've kept existing select list in the subquery for the count
if ORDER BY is given. But it had accidentally affect to GROUP BY
queries also. It should keep the previous behavior in that case.

Fixes #30886.

I don't think this is a good abstraction because the internal method is
used only if the relation need to be applied join dependency.

When using a has_many through relation and then summing an attribute
the distinct was not being used. This will ensure that when summing
an attribute, the number is only used once when distinct has been used.

A common source of bugs and code bloat within Active Record has been the
need for us to maintain the list of bind values separately from the AST
they're associated with. This makes any sort of AST manipulation
incredibly difficult, as any time we want to potentially insert or
remove an AST node, we need to traverse the entire tree to find where
the associated bind parameters are.

With this change, the bind parameters now live on the AST directly.
Active Record does not need to know or care about them until the final
AST traversal for SQL construction. Rather than returning just the SQL,
the Arel collector will now return both the SQL and the bind parameters.
At this point the connection adapter will have all the values that it
had before.

A bit of this code is janky and something I'd like to refactor later. In
particular, I don't like how we're handling associations in the
predicate builder, the special casing of `StatementCache::Substitute` in
`QueryAttribute`, or generally how we're handling bind value replacement
in the statement cache when prepared statements are disabled.

This also mostly reverts #26378, as it moved all the code into a
location that I wanted to delete.

/cc @metaskills @yahonda, this change will affect the adapters

Fixes #29766.
Fixes #29804.
Fixes #26541.
Close #28539.
Close #24769.
Close #26468.
Close #26202.

There are probably other issues/PRs that can be closed because of this
commit, but that's all I could find on the first few pages.

Since #26972, `ORDER BY` is kept if `LIMIT` is presented for
performance. But in most SQL servers (e.g. PostgreSQL, SQL Server, etc),
`ORDER BY` expressions must appear in select list for `SELECT DISTINCT`.
We should not replace existing select list in that case.

The `find_each`, `find_in_batches` and `in_batches` APIs usually operate
on large numbers of records, where it's preferable not to load them all
into memory at once.

If the query cache is enabled, it will hold onto the query results until
the end of the execution context (request/job), which means the memory
used is still proportional to the total number of records. These queries
are typically not repeated, so the query cache isn't desirable here.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

The delegation was needed since passing `relation` with
`relation.bound_attributes`. It should use `relation.arel` in that case.

Deprecate passing arguments and block at the same time to `count` and `sum` in `ActiveRecord::Calculations`

`select`, `count`, and `sum` in `Relation` are also `Enumerable` method
that can be passed block. `select` with block already doesn't take
arguments since 4fc3366d. This is follow up of that.

The argument of `Arel::SelectManager.new` is `table`, not `engine`.

https://github.com/rails/arel/blob/v8.0.0/lib/arel/select_manager.rb#L10

Follow up of #24203.

Since b644964b `ActiveRecord::Relation` includes `Enumerable` so it is
enough to call `super` simply.

When a grouped calculation contains a having clause that references a
selected value, we need to include that selected value in the query.

Postgres doesn't support referencing a selected value in a having
clause, but other databases do; we can skip the test on the pg adapter
but run it for the others.

This was fixed before in 9a298a16, but
the test coverage was lost in 5a05207d.
The fix regressed in 6311975f and was
removed in 97d46c17.

`select_values` is a local variable defined at previous line.
`select_values += select_values` is totally useless.