Re: does oracle us both block and row level nested loop joins?

From: Tanel Põder <tanel.poder.003_at_mail.ee>
Date: Fri, 30 Jul 2004 19:04:11 +0300
Message-ID: <07f701c4764e$da945f60$36ae9fd9@porgand>

> I'm fairly certain that Oracle uses row level nested loop joins when
tables are indexes.
> However, does it do 'block' level nested loop joins to improve performance
when
> tables are not indexed. I do not know how to test for this.

Try this test:

create table t1(a, b) as select rownum, cast('x' as char(1000)) from sys.obj$ where rownum <=50;
create table t2(a, b) as select rownum, cast('x' as char(1000)) from sys.obj$ where rownum <=500;

--create index i1 on t1(a);
--create index i2 on t2(a);

analyze table t1 compute statistics;
analyze table t2 compute statistics;

select table_name, blocks from user_tables where table_name in ('T1', 'T2');

set autot trace exp stat

select /*+ USE_NL(t1, t2) */ t1.*, t2.* from t1, t2 where t1.a = t2.a;

set autot off

You see from consistent gets that all of the blocks in T2 are opened 50 times (total LIOs = the amount of rows in T1 * blocks in T2 + some overhead).

At least this experiment shows that there is no block prefetching or row caching used and all joining is done row-by-row. About indexed NL joins and prefetching, uncomment the above create index clauses one by one and see the results then (there was a quite interesting thread about index prefetching in NL joins couple of months ago here..)

Tanel.

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Received on Fri Jul 30 2004 - 11:01:35 CDT