在PL-SQL中拆分多个表的联接

Question

I need to create a function that returns a sysref cursor pointing to some result set. I need to join a global temporary table with several other tables, the basic aproach would look like this:

 Open sysrefcur for
     select *
     from
       (select * 
       from globaltemptable) t1
       left join t2
         on <conditions1>
       left join t3
         on <conditions2>
       left join t4
         on <conditions2>
       ...
       ;
       return sysrefcur;

The problem is that I have to joint the global temporary table (GTT) with around 30 additional tables (I know, it's a lot) but DBAs set a limit of 10 joins,

So I was thinking of joining the GTT table with a few tables, inserting the result in another GTT, join it to another few tables and so on.

I want to know if theres a better approach to this and if I can avoid creating additional temporary tables at all.

Thanks for your help.

Answer 1

1. Create views that do 5 joins each and then join 6 views.

2. Use CTE. Define with statements with some joins then join results of with statements.

Answer 2

I think the approach you suggested in your post is probably best. It is simple to implement and understand, not horribly less efficient than doing this the right way (I'll second/third the criticisms of your DBA's policy), and it clearly conforms to the policy. (Using views or CTEs are arguably "violations", since, at end of it all, it's a 30-table query still).

and if I can avoid creating additional temporary tables at all.

You can avoid creating an additional global temp table if you are able to add placeholder columns to the one you have. In that case, your solution would look something like this:

ALTER TABLE globaltemptable ADD ... placeholder columns for T2, T3, T4, etc data...


MERGE INTO globaltemptable t
USING ( SELECT * FROM globaltemptable t1
        LEFT JOIN ... t2 through t10  (10 table join) ...
      ) u
ON ( t.primary_key = u.primary_key )
WHEN MATCHED THEN UPDATE SET ... t2 through t10 columns in globaltemptable ...
;

... repeat merge for tables t11 through t19

... repeat merge for tables t20 through t28

... repeat merge for tables t29 through t30


OPEN SYSREFCUR FOR SELECT * FROM globaltemptable -- no joins.