Use FETCH FIRST for Oracle12 and test ROWNUM <= for Oracle 11g or older version to test sql limit behavior

[yahonda]

Summary

This pull request addresses this following failure when tested with Oracle Database 12c.

$ cd activerecord
$ ARCONN=oracle bundle exec ruby -W -w -I"lib:test" test/cases/finder_test.rb -n test_take_and_first_and_last_with_integer_should_use_sql_limit

... snip ...

# Running:

F

Finished in 0.826707s, 1.2096 runs/s, 1.2096 assertions/s.

  1) Failure:
FinderTest#test_take_and_first_and_last_with_integer_should_use_sql_limit [test/cases/finder_test.rb:568]:
Query pattern(s) /LIMIT|ROWNUM <=/ not found.
Queries:
SELECT  "TOPICS".* FROM "TOPICS" FETCH FIRST :a1 ROWS ONLY

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
$

Since Oracle 12c database introduces better top N query using FETCH FIRST n ROWS and Arel supports this syntax

Other Information

* Rails `master` branch
* Oracle enhanced adapter `rails51` branch
* Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
* ruby 2.4.0dev (2016-08-04 trunk 55812) [x86_64-linux]

[rails-bot]

r? @matthewd

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[yahonda]

Added another similar case and squashed.

[senny]

@yahonda should we check for both or are previous versions not compatible with Rails 5?

[yahonda]

@senny Thanks for the review.
Old behaviour is still compatible with Rails 5. Let me update my commits.

[yahonda]

Add Oracle 11g support then squashed. Somehow it gets an error at ActionCable https://travis-ci.org/rails/rails/jobs/150643333

[senny]

@yahonda thanks. I'll backport it to 5-0-stable as well.