RE: Oracle set operator (UNION, MINUS) and read consistency

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Tue, 17 Nov 2015 19:20:50 +0000
Message-ID: <CE70217733273F49A8A162EE074F64D9282B16E0_at_EXMBX01.thus.corp>

I don't think your example proves the point - it may simply be demonstrating that UNION ALL queries operate from the bottom up, and that function calls in the select list operate after column projection.

(I don't believe, that, by the way, but it is an alternative explanation for your result.)

Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
_at_jloracle

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From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] on behalf of Iggy Fernandez [iggy_fernandez_at_hotmail.com] Sent: 17 November 2015 18:50
To: Oracle-L Freelists
Subject: RE: Oracle set operator (UNION, MINUS) and read consistency

The assumption behind this question is that UNION ALL (and other set operators) dictate that Oracle process the branches in linear fashion as dictated by the order in which they appear in the text. That assumption is incorrect. For example, "join factorization" may produce an unexpected query plan. See https://blogs.oracle.com/optimizer/entry/optimizer_transformations_join_factorization. As another example, Oracle is capable of processing the branches in parallel. See https://docs.oracle.com/database/121/VLDBG/GUID-1F4C90F9-3EF5-423A-B55B-2593FB3F1433.htm.

Here is a demonstration that indicates that read consistency applies to the entire statement not just to individual branches. The demonstration uses an autonomous transaction.

SQL*Plus: Release 11.2.0.1.0 Production on Tue Nov 17 13:39:26 2015

Copyright (c) 1982, 2009, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> create or replace function autonomous return integer as   2 pragma autonomous_transaction;
  3 begin
  4 dbms_lock.sleep(1);
  5 insert into dummy select sysdate from dual;   6 commit;
  7 return 1;
  8 end;
  9 /

Function created.

SQL>
SQL> drop table dummy;

Table dropped.

SQL>
SQL> create table dummy as
  2 select sysdate as currenttime from dual where 1=2;

Table created.

SQL>
SQL> insert into dummy values (sysdate);

1 row created.

SQL> commit;

Commit complete.

SQL>
SQL> alter session set nls_date_format = 'hh:mm:ss';

Session altered.

SQL>
SQL> select * from dummy;

CURRENTT

---

01:11:26

SQL>
SQL> select 1, autonomous(), d.* from dummy d   2 union all
  3 select 2, null, d.* from dummy d;

         1 AUTONOMOUS() CURRENTT
---------- ------------ --------

         1            1 01:11:26
         2              01:11:26

SQL>
SQL> select * from dummy;

CURRENTT

---

01:11:26
01:11:27

SQL>
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production With the Partitioning, OLAP, Data Mining and Real Application Testing options

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Date: Tue, 17 Nov 2015 17:46:11 +0000
From: dmarc-noreply_at_freelists.org
To: oracle-l_at_freelists.org
Subject: Oracle set operator (UNION, MINUS) and read consistency

Hi list,

I have a query using set operators like the following

select * from <table 1>
union all
select * from <table 2>

Does Oracle consider this as ONE query which allows both 'select' statements have read consistency back to the time when the first 'select' statement starts?

Thanks,
Lei

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http://www.freelists.org/webpage/oracle-l Received on Tue Nov 17 2015 - 20:20:50 CET