Make Dataset#get and #first without argument not create intermediate datasets if receiver uses raw SQL

There's no reason for a dataset clone in this case, as the
limit and select settings will be ignored.

This also fixes use of Dataset#get without argument when using the
implicit_subquery extension. Previously, it would try to use a
subquery in an unhelpful way:

  DB['SELECT a FROM b WHERE c = 1'].extension(:implicit_subquery).get
  #  SELECT NULL AS v FROM (SELECT a FROM b WHERE c = 1) AS t1 LIMIT 1

Now, it runs the query directly and returns the first value.

Author: jeremyevans

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Make Dataset#get and #first without argument not create intermediate datasets if receiver uses raw SQL (jeremyevans)
+
 * Add dataset_run extension, for building SQL using datasets, and running with Database#run (jeremyevans)
 
 * Switch default connection pool to timed_queue on Ruby 3.2+ (jeremyevans)

lib/sequel/dataset/actions.rb

@@ -217,7 +217,7 @@ def first(*args, &block)
       case args.length
       when 0
         unless block
-          return single_record
+          return(@opts[:sql] ? single_record! : single_record)
         end
       when 1
         arg = args[0]
@@ -282,13 +282,21 @@ def first!(*args, &block)
     #
     #   DB[:table].get{[sum(id).as(sum), name]} # SELECT sum(id) AS sum, name FROM table LIMIT 1
     #   # => [6, 'foo']
+    #
+    # If called on a dataset with raw SQL, returns the
+    # first value in the dataset without changing the selection or setting a limit:
+    #
+    #   DB["SELECT id FROM table"].get # SELECT id FROM table
+    #   # =>  3
     def get(column=(no_arg=true; nil), &block)
       ds = naked
       if block
         raise(Error, 'Must call Dataset#get with an argument or a block, not both') unless no_arg
         ds = ds.select(&block)
         column = ds.opts[:select]
         column = nil if column.is_a?(Array) && column.length < 2
+      elsif no_arg && opts[:sql]
+        return ds.single_value!
       else
         case column
         when Array

spec/core/dataset_spec.rb

@@ -3035,6 +3035,12 @@ def supports_cte_in_subselect?; false end
     Sequel.mock[:t].first.must_be_nil
   end
 
+  it "should return first record in query when using raw SQL" do
+    db = Sequel.mock(:fetch=>{:v=>1})
+    db['SELECT 1'].first.must_equal(:v=>1)
+    db.sqls.must_equal ['SELECT 1']
+  end
+  
   it "#last should raise if no order is given" do
     proc {@d.last}.must_raise(Sequel::Error)
     proc {@d.last(2)}.must_raise(Sequel::Error)
@@ -3375,7 +3381,9 @@ def supports_cte_in_subselect?; false end
     @d.with_sql('SELECT foo').get(:name).must_equal "SELECT foo"
     @d = @d.with_fetch(:name=>1, :abc=>2)
     @d.with_sql('SELECT foo').get{[name, n[abc]]}.must_equal [1, 2]
-    @d.db.sqls.must_equal ['SELECT foo'] * 2
+    @d = @d.with_fetch(:name=>1)
+    @d.with_sql('SELECT foo').get.must_equal 1
+    @d.db.sqls.must_equal ['SELECT foo'] * 3
   end
 
   it "should handle cases where no rows are returned" do