use subquery for limit when DISTINCT is used (in Oracle)

2 files changed, 14 insertions, 2 deletions

diff --git a/lib/arel/visitors/oracle.rb b/lib/arel/visitors/oracle.rb
index 659b75f16d..c691ce294e 100644
--- a/lib/arel/visitors/oracle.rb
+++ b/lib/arel/visitors/oracle.rb
@@ -6,7 +6,9 @@ module Arel
       def visit_Arel_Nodes_SelectStatement o
         o = order_hacks(o)
 
-        if o.limit && o.orders.empty? && !o.offset
+        # if need to select first records without ORDER BY and GROUP BY and without DISTINCT
+        # then can use simple ROWNUM in WHERE clause
+        if o.limit && o.orders.empty? && !o.offset && o.cores.first.projections.first !~ /^DISTINCT /
           o.cores.last.wheres.push Nodes::LessThanOrEqual.new(
             Nodes::SqlLiteral.new('ROWNUM'), o.limit
           )
@@ -31,7 +33,7 @@ module Arel
           eosql
         end
 
-        if o.limit && !o.orders.empty?
+        if o.limit
           o       = o.dup
           limit   = o.limit
           o.limit = nil
diff --git a/spec/visitors/oracle_spec.rb b/spec/visitors/oracle_spec.rb
index cdf0e3427f..93ade3ef3c 100644
--- a/spec/visitors/oracle_spec.rb
+++ b/spec/visitors/oracle_spec.rb
@@ -71,6 +71,16 @@ module Arel
             }
           end
 
+          it 'creates a subquery when there is DISTINCT' do
+            stmt = Nodes::SelectStatement.new
+            stmt.cores.first.projections << Nodes::SqlLiteral.new('DISTINCT id')
+            stmt.limit = 10
+            sql = @visitor.accept stmt
+            sql.should be_like %{
+              SELECT * FROM (SELECT DISTINCT id) WHERE ROWNUM <= 10
+            }
+          end
+
           it 'creates a different subquery when there is an offset' do
             stmt = Nodes::SelectStatement.new
             stmt.limit = 10