Re: Method to pass data between queries in a multi-statement transaction

One can argue here that we can use the results from table3 to perform the
delete operation instead of joining again. But table3 may be subject to
updates which makes the situation tricky.

On Wed, Apr 17, 2019 at 8:30 PM Souvik Bhattacherjee <kivuosb(at)gmail(dot)com>
wrote:

> There are few if any situations where you need to immediately and
> completely pass all values from one query to another in the same
> transaction where the queries cannot just be combined into a single
> statement. Your representative example is one that is easily combined into
> a single statement.
>
> > What if I need the result of the join to be stored into table3 as well
> as the tuples that participated in the query to be deleted from table1. The
> following can be done without the need to transfer values from the previous
> query into the next:
>
> begin;
> insert into table3 (id, attr1, attr2) (select t1.cid, t1.empname, t2.dept
> from table1 t1, table2 t2 where t1.cid = t2.cid);
> delete from table1 where cid in (select c.cid from table1 t1, table2 t2
> where t1.cid = t2.cid);
> commit;
>
> However note that we have to perform the join twice, which is not
> efficient. Now to make things worse, increase the number of tables to join
> while imposing the requirement of tuple deletion to apply to all or to a
> subset of the tables that participate in join.
>
> Now, the stuff you are trying seems to indicate you are trying to do
> something in C, inside the engine itself, with all of this. If that is the
> case you may want to be more clear as to what you are attempting to do.
> But as far as server SQL goes the only persistence area are
> tables/relations - including temporary ones.
>
> > I'm trying to modify the engine here.
>
> -SB
>
> On Wed, Apr 17, 2019 at 6:16 PM David G. Johnston <
> david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
>
>>
>> On Wed, Apr 17, 2019 at 3:04 PM Souvik Bhattacherjee <kivuosb(at)gmail(dot)com>
>> wrote:
>>
>>> Hello,
>>>
>>> I'm trying to pass some values between queries in a multi-statement
>>> transaction. For example, consider the following representative
>>> multi-statement transaction:
>>>
>>> begin;
>>> select * from table1 t1, table2 t2 where t1.cid = t2.cid;
>>> delete from table1 where cid in
>>> (values-to-be-populated-from-the-previous-query);
>>> commit;
>>>
>>
>> There are few if any situations where you need to immediately and
>> completely pass all values from one query to another in the same
>> transaction where the queries cannot just be combined into a single
>> statement. Your representative example is one that is easily combined into
>> a single statement.
>>
>> Now, the stuff you are trying seems to indicate you are trying to do
>> something in C, inside the engine itself, with all of this. If that is the
>> case you may want to be more clear as to what you are attempting to do.
>> But as far as server SQL goes the only persistence area are
>> tables/relations - including temporary ones.
>>
>> David J.
>>
>