On 12/21/11 3:59 AM, JulioSerje wrote:
Hi Rick,
Unfortunately our expressions would require a high number of subqueries;
ours is an engineering application that require many values from several
tables in order to compute multiple complex formulae.
In our specific case we know that by design that our UPDATE ..FROM
statements will compute the same as the statement based on subqueries (i.e.
statements are not ambiguous at all).
The question is, are you aware of any plans to implement MERGE in Derby? In
the meanwhile we are considering the use of an intermediate table with
results INSERTed, as insert can use a full fledged select clause, and later
use a much simpler UPDATE statement with less and simpler subqueries.
Hi Julio,
There is an existing enhancement request for the MERGE operator:
https://issues.apache.org/jira/browse/DERBY-3155 If you add your vote on
that issue, you will increase its chances of being implemented.
Hope this helps,
-Rick
Thanks again,
Julio
Are there any plans to
Rick Hillegas-3 wrote:
Hi Julio,
I haven't looked into the details of what the Derby optimizer does in
the case of multiple SET clauses, each driven by its own subselect. It
is likely that this is an underoptimized case. If you rewrite your query
to use subselects in the SET clauses, and you experience poor
performance on Derby, you may be able to get advice on this list about
how to improve that performance.
It is possible that the Derby UDPATE will compute different results than
the UPDATE...FROM statements in your other databases. Derby may even
raise an error. If Derby raises an error, this may be a warning to you
that your UPDATE...FROM statements are ambiguous. An UPDATE...FROM
statement may end up updating the same column multiple times in a
non-deterministic way. This can happen if the join in the FROM clause
returns multiple joined rows for each row in the table you are updating.
In this situation, the standard syntax will raise an error because the
subselects are supposed to return one row each. More information on this
problem can be found here:
http://sqlblog.com/blogs/hugo_kornelis/archive/2008/03/10/lets-deprecate-update-from.aspx
Hope this is helpful,
-Rick
