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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: Updating rows with an open cursor...what is the expected behavior?
Date Tue, 12 Jun 2007 19:11:45 GMT
Some history on updatable result sets.  I think all this comes from 
pre-derby code - I am not sure if it is represented in the comments but
might help with understanding existing code decisions.  Also this is
just from memory of changes I was not involved with.

I think we use to try and avoid index scans because of the kinds of
issues you are seeing.

1) At one point the code did not allow updates to a result set unless the
select statement had a for update clause.

2) If a select had a for update clause then we did not allow an index scan
if any column in the for update clause was in the index.  Note this 
meant if no column listed then all assumed updatable and thus no index
scan allowed.

Then we found too many applications assumed all select's could be 
updatable, so we defaulted a result set to updatable.  I don't remember 
if this was an actual standard or just a defacto standard by actual J2SE 
  app server usage.   But with rules 1
and 2 it meant that almost always we could not use an index which led
to unacceptable performance.  So we allowed indexes to be used when
no for update clause is specified.  I am actually not sure what we
do today if you specify the for update clause.

Army wrote:
> While investigating DERBY-2805 I was trying to understand the purpose of 
> "ResultSetNode.markOrderingDependent()" as it is used in the 
> language/optimizer layer of code.  In particular, see 
> IndexRowToBaseRowNode:
> 
>     /**
>      * Notify the underlying result set tree that the result is
>      * ordering dependent.  (For example, no bulk fetch on an index
>      * if under an IndexRowToBaseRow.)
>      */
>     void markOrderingDependent()
>     {
>         /* NOTE: We use a different method to tell a FBT that
>          * it cannot do a bulk fetch as the ordering issues are
>          * specific to a FBT being under an IRTBR as opposed to a
>          * FBT being under a PRN, etc.
>          */
>         source.disableBulkFetch();
>     }
> 
> As part of some testing I ran statements in ij which effectively do the 
> following:
> 
>   1. Get a SELECT cursor on the table that specifies an ORDER BY
>   2. In the middle of iterating through the table, update one of the rows.
>   3. Continue iterating through the cursor.
> 
> To my surprise, the results of the query differs depending on whether 
> Derby does an Index Scan or a Table Scan on the underlying table.  In 
> the case of an Index Scan the open cursor will see the updated row; but 
> in the case of a Table Scan the cursor will *not* see the updated row.  
> A quick look at the query plan shows that in both cases Derby is doing 
> "share row locking".
> 
> To see this in ij, do the following (I haven't tried this in JDBC yet...):
> 
> autocommit off;
> create table str(c1 int, c2 int, c3 int);
> insert into str values (1, 1, 2), (1, 2, 3), (1, 3, 4), (1, 4, 5);
> create index str1 on str(c1, c2);
> commit;
> 
> -- Force index scan.
> 
> get cursor c1 as
>   'select c2, c3 from str --DERBY-PROPERTIES index=str1
>    where c1 = 1 order by c2';
> 
> next c1;  -- returns "1, 2"
> update str set c2 = 4 where c2 = 2;
> next c1;  -- returns "3, 4"
> next c1;  -- returns "4, 3" ==> updated row is picked up
> next c1;  -- returns "4, 5"
> next c1;  -- no current row
> close c1;
> rollback;
> 
> -- Force table scan.
> 
> get cursor c1 as
>   'select c2, c3 from str --DERBY-PROPERTIES index=null
>    where c1 = 1 order by c2';
> 
> next c1;  -- returns "1, 2"
> update str set c2 = 4 where c2 = 2;
> next c1;  -- returns "2, 3" ==> updated row is *not* picked up
> next c1;  -- returns "3, 4"
> next c1;  -- returns "4, 5"
> next c1;  -- no current row
> close c1;
> rollback;
> 
> Is there a reason we get different results for the two query plans, or 
> is this a bug?  If it's a bug, then what is the correct behavior for 
> this kind of thing?
> 
> Note that if I comment out the call to "disableBulkFetch()" in the code 
> shown at the top of this mail, then the order of the rows changes for 
> the Index Scan (they are no longer in correct order), but the open 
> cursor still picks up the updated row.
> 
> I plan to see if I can reproduce this behavior using a JDBC program, but 
> if anyone has any thoughts/insight in the meantime, I'd appreciate the 
> pointers. And apologies in advance if I'm missing something obvious...
> 
> Army
> 
> 

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