RE: Tuning Update query

Do you have handy the script or load used to create GTT_TAB?  

It seems passing strange that you want to debug what is essentially a row by row case computation instead of creating the disjoint sets of rows to operate on as a bulk set.  

Consider GTT_TAB_XX_Y alias XX_Y, which contains only TMP.CIND=’Y’ and TMP.PART_DT for which the corresponding TMP.PART_DT = P.PART_DT has P.CODE=’XX’.  

Then insert into GTT_TAB  

Select p.<the column list from P>, xx_y.<the column list from XX_Y> where p.tid=xx_y.tid and p.part_dt=xx_y.part_id  

All the rows from xx_y are used and should be a full table scan, matching against the presumably indexed p.tid, p.partid from P.  

Do this for each of your separate “updates”, appending into GTT_TAB, and directly append the disjoint bits that require no update directly into GTT_TAB however you do now. The only tricky set is P.CODE=’AA’ for which you have two updates. For that, use GTT_TAB_AA_5 gets appended into GTT_TAB_AA_6 and GTT_TAB_AA_6 gets appended into GTT_TAB.  

All the other rows (which meet zero of your update predicates) you dump directly into GTT_TAB from wherever you are currently plucking them.  

This of course fixes your ‘Y’ problem and operates set wise using bulk inserts AND operates only on rows that need attention instead of filtering out the rows you’re not interested in for each update.  

Write out the differences as a dataflow diagram to understand why this always wins.  

mwf  

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Jonathan Lewis Sent: Tuesday, May 04, 2021 5:07 AM
To: Lok P
Cc: Oracle L
Subject: Re: Tuning Update query    

1. I wasn't suggesting that you use an updatable join view, but I can see how you could misinterpret what I said. The idea was to see if you could write a single subquery that joined several of the source tables (such as PRODUCT_TAB) and then still carry out that full tablescan of the GTT but using that join as your subquery update rather than having "one tablescan and update = one subquery".

I see you're already doing something of this kind using the MERGE command; if you're getting an ORA-01779 when you try to rewrite one of your updates as an update through an updatable join view this means that equivalent code to do a MERGE could produce a run-time error ORA-30926). The update portion of a merge and an updatable join view both require the same uniqueness but the view enforces the logical requirement at compile time while the merge command allows you to get away with the update so long as you get lucky with the data.  

I note that you still haven't moved the "tmp.cind = 'Y'" predicate to the correct position - that should have been the zeroth step before worrying about anything else. I assume the one update I picked out isn't the only one where this error occurs.    

b) Correct regarding multiple GTTs. Each "insert /*+ append */" would probably require it's own target GTT. Remember, though, that I haven't examined your original posting in detail, and you probably haven't told us everything a consultant would ask about so if your current code has some parts that update disjoint subsets of the data you might find parts of the rewrite where you could do multiple inserts into the same GTT.  

One thing to bear in mind - a possible bar to adopting this approach - is that you would have to commit after insert otherwise the next insert, or the next query against the target GTT would raise the (unexpectedly parallel) error: ORA-12838: cannot read/modify an object after modifying it in parallel  

Regards

Jonathan Lewis          

On Mon, 3 May 2021 at 12:58, Lok P <loknath.73_at_gmail.com> wrote:

Thank You Jonathan and Lothar. I was trying to modify one of the sample updates(as below) as per your suggested method.  

UPDATE GTT_TAB TMP   SET (c1, c2, c3, c4,c5.. c11) =

         ( (SELECT col1,col2,..col11

              FROM PRODUCT_TAB P              WHERE P.TID = TMP.TID

                   AND TMP.CIND = 'Y'

                   AND P.PART_DT = TMP.PART_DT

                   AND P.CODE = 'XX')) 

 

Jonathan, let me know if my understanding is correct on the suggested points

" a) See if you can minimise the number of update ste[ps by updating from a JOIN of several source tables"
 

So if I get the above point correct then I was trying to modify the UPDATE as below , but I am getting ORA-01779 while running to see the plan. So does it mean that the GTT has to have a unique key present in it to have this method work?  

UPDATE (SELECT P.COL1...P.col11,TMP.C1.. TMP.C11

              FROM PRODUCT_TAB P,GTT_TAB TMP              WHERE P.TID = TMP.TID

                   AND TMP.CIND = 'Y'

                   AND P.PART_DT = TMP.PART_DT

                   AND P.CODE = 'XX'

)      SET    C1=COL1,    ...     C11=COL11;

ERROR at line 10:

ORA-01779: cannot modify a column which maps to a non key-preserved table  

Regarding below point ,

"b) Rewrite the code to step through a series of GTT doing

        insert /*+ append */ into next_gtt select from previous_gtt join {source tables}"  

Do you mean i have to replace the UPDATE with INSERT /*+APPEND*/... queries but as it cant be done in the same GTT , so i have to create multiple GTT's for each UPDATES, so as to replace them with INSERT APPEND queries?    

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