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
ProductionWith 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--------01:11:2601:11:27
SQL>SQL> exitDisconnected from Oracle Database 11g Enterprise Edition Release
11.2.0.4.0 - 64bit ProductionWith the Partitioning, OLAP, Data Mining and Real
Application Testing options
Date: Tue, 17 Nov 2015 17:46:11 +0000
From: dmarc-noreply@xxxxxxxxxxxxx
To: oracle-l@xxxxxxxxxxxxx
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 allselect * 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
