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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