Re: Calculation in update query

Christopher A. Goodfellow wrote:
 > So I do need to read (select) the value first, perform the calculation, and
 > then update the value?  I just wanted to be able to set the value based on
 > the current value in one step.  There are 10's of thousands of calculations
 > that will be performed in this loop.  Three steps (Select, Calculate,
 > Update) are just longer than one.  Thanks.

Ah, sorry, I misunderstood you.  I thought you were
concerned about the possibility of concurrent updates,
i.e. the necessity of locking.  That's why my brain
immediately went into the SELECT FOR UPDATE direction.
:-)

Well, yes, of course you can update a column based on
the old value of that column.  The UPDATE command
accepts arbitrary expressions for the new value which
may reference the old one.  There's even an example
which does exactly that (on the PostgreSQL's doc page
on the UPDATE command):

http://www.postgresql.org/docs/7.4/static/sql-update.html

Best regards
   Oliver

--
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
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