Create Arel::Visitors::Oracle12 to provide better top-N query

to support `FETCH FIRST n ROWS` and `OFFSET` for Oracle 12c database

2 files changed, 54 insertions, 0 deletions

diff --git a/lib/arel/visitors.rb b/lib/arel/visitors.rb
index 4a8d254ba7..f492ca2d9d 100644
--- a/lib/arel/visitors.rb
+++ b/lib/arel/visitors.rb
@@ -6,6 +6,7 @@ require 'arel/visitors/postgresql'
 require 'arel/visitors/mysql'
 require 'arel/visitors/mssql'
 require 'arel/visitors/oracle'
+require 'arel/visitors/oracle12'
 require 'arel/visitors/where_sql'
 require 'arel/visitors/dot'
 require 'arel/visitors/ibm_db'
diff --git a/lib/arel/visitors/oracle12.rb b/lib/arel/visitors/oracle12.rb
new file mode 100644
index 0000000000..4a42343c9b
--- /dev/null
+++ b/lib/arel/visitors/oracle12.rb
@@ -0,0 +1,53 @@
+module Arel
+  module Visitors
+    class Oracle12 < Arel::Visitors::ToSql
+      private
+
+      def visit_Arel_Nodes_SelectStatement o, collector
+        # Oracle does not allow LIMIT clause with select for update
+        if o.limit && o.lock
+          o = o.dup
+          o.limit = []
+        end
+
+        super
+      end
+
+      def visit_Arel_Nodes_SelectOptions o, collector
+        collector = maybe_visit o.offset, collector
+        collector = maybe_visit o.limit, collector
+        collector = maybe_visit o.lock, collector
+      end
+
+      def visit_Arel_Nodes_Limit o, collector
+        collector << "FETCH FIRST "
+        collector = visit o.expr, collector
+        collector << " ROWS ONLY"
+      end
+
+      def visit_Arel_Nodes_Offset o, collector
+        collector << "OFFSET "
+        visit o.expr, collector
+        collector << " ROWS"
+      end
+
+      def visit_Arel_Nodes_Except o, collector
+        collector << "( "
+        collector = infix_value o, collector, " MINUS "
+        collector << " )"
+      end
+
+      def visit_Arel_Nodes_UpdateStatement o, collector
+        # Oracle does not allow ORDER BY/LIMIT in UPDATEs.
+        if o.orders.any? && o.limit.nil?
+          # However, there is no harm in silently eating the ORDER BY clause if no LIMIT has been provided,
+          # otherwise let the user deal with the error
+          o = o.dup
+          o.orders = []
+        end
+
+        super
+      end
+    end
+  end
+end