Re: How to use read uncommitted transaction level and set update order

Christophe Pettus <xof(at)thebuild(dot)com> writes:
> On Dec 19, 2009, at 11:24 AM, Andrus wrote:
>> update test1 set a=4, b=a ;

>> How to use updated value ?

> The problem here isn't the transaction isolation level. The order of
> evaluation in an UPDATE statement is (for practical purposes):
> Evaluate all of the right-hand side expressions, and then assign them
> all to the left-hand side fields.

This is required by the SQL standard, and always has been --- I quote SQL92:

6) The <value expression>s are effectively evaluated before updat-
ing the object row. If a <value expression> contains a reference
to a column of T, then the reference is to the value of that
column in the object row before any value of the object row is
updated.

I would be quite surprised if there are any SQL databases that do this
differently.

regards, tom lane