Abandon TOP support.

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 1a246f71616cf246a75ef6cbdb56032e43d4e643.

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 be48ed3071fd6524d0145c4ad3faeb4aafe3eda3.
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.

4 files changed, 0 insertions, 16 deletions

diff --git a/activerecord/lib/arel/visitors/depth_first.rb b/activerecord/lib/arel/visitors/depth_first.rb
index bcf8f8f980..5948622aea 100644
--- a/activerecord/lib/arel/visitors/depth_first.rb
+++ b/activerecord/lib/arel/visitors/depth_first.rb
@@ -34,7 +34,6 @@ module Arel # :nodoc: all
         alias :visit_Arel_Nodes_Ordering          :unary
         alias :visit_Arel_Nodes_Ascending         :unary
         alias :visit_Arel_Nodes_Descending        :unary
-        alias :visit_Arel_Nodes_Top               :unary
         alias :visit_Arel_Nodes_UnqualifiedColumn :unary
 
         def function(o)
diff --git a/activerecord/lib/arel/visitors/dot.rb b/activerecord/lib/arel/visitors/dot.rb
index d352b81914..76830412d4 100644
--- a/activerecord/lib/arel/visitors/dot.rb
+++ b/activerecord/lib/arel/visitors/dot.rb
@@ -81,7 +81,6 @@ module Arel # :nodoc: all
         alias :visit_Arel_Nodes_Not               :unary
         alias :visit_Arel_Nodes_Offset            :unary
         alias :visit_Arel_Nodes_On                :unary
-        alias :visit_Arel_Nodes_Top               :unary
         alias :visit_Arel_Nodes_UnqualifiedColumn :unary
         alias :visit_Arel_Nodes_Preceding         :unary
         alias :visit_Arel_Nodes_Following         :unary
diff --git a/activerecord/lib/arel/visitors/mssql.rb b/activerecord/lib/arel/visitors/mssql.rb
index 9aedc51d15..d564e19089 100644
--- a/activerecord/lib/arel/visitors/mssql.rb
+++ b/activerecord/lib/arel/visitors/mssql.rb
@@ -12,13 +12,6 @@ module Arel # :nodoc: all
 
       private
 
-        # `top` wouldn't really work here. I.e. User.select("distinct first_name").limit(10) would generate
-        # "select top 10 distinct first_name from users", which is invalid query! it should be
-        # "select distinct top 10 first_name from users"
-        def visit_Arel_Nodes_Top(o)
-          ""
-        end
-
         def visit_Arel_Visitors_MSSQL_RowNumber(o, collector)
           collector << "ROW_NUMBER() OVER (ORDER BY "
           inject_join(o.children, collector, ", ") << ") as _row_num"
diff --git a/activerecord/lib/arel/visitors/to_sql.rb b/activerecord/lib/arel/visitors/to_sql.rb
index 0682c066fb..81ca63f261 100644
--- a/activerecord/lib/arel/visitors/to_sql.rb
+++ b/activerecord/lib/arel/visitors/to_sql.rb
@@ -237,8 +237,6 @@ module Arel # :nodoc: all
         def visit_Arel_Nodes_SelectCore(o, collector)
           collector << "SELECT"
 
-          collector = maybe_visit o.top, collector
-
           collector = maybe_visit o.set_quantifier, collector
 
           collect_nodes_for o.projections, collector, SPACE
@@ -405,11 +403,6 @@ module Arel # :nodoc: all
           visit o.expr, collector
         end
 
-        # FIXME: this does nothing on most databases, but does on MSSQL
-        def visit_Arel_Nodes_Top(o, collector)
-          collector
-        end
-
         def visit_Arel_Nodes_Lock(o, collector)
           visit o.expr, collector
         end