When does oracle use a complete nested loop join?

Hi!
Does Oracle Database use a complete nested loop join? I mean, imagine 2 tables without any indexes.. is there any case where for each row in the outer table Oracle does a complete scan in the inner table? I know that this is the original algorithm for the nested loop join, but some data bases prefer to make a temp table to autoindex the inner table and never makes the complete scan in the inner table..
thanks!!

user12040235 wrote:
If the table do not have indexes.. some data bases prefer to scan one time the inner table, to index all values, and than, for every row in the outter loop table, it will do a index search.
I just like to know oracle does the same thing, or it does the complete scan..If you have two tables without indexes, Oracle may consider scanning one table, extracting the smallest data set it can get away with, and then building a hash table of that data set (rather than creating an in-memory copy with index). At this point Oracle can then do a nested loop join into the in-memory hash table.
However, this is called a hash join, and the order of tables will appear to be reversed, viz:
nested loop
    table scan full ABC
    table scan full XYZ
{code]
becomeshash join
table scan full XYZ
table scan full ABC
See: http://jonathanlewis.wordpress.com/2010/08/02/joins/ as a starting point if you want to read more on this topic.
Regards
Jonathan Lewis

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Generally when does optimizer use nested loop and Hash joins ?

Version: 11.2.0.3, 10.2
Lets say I have a table called ORDER and ORDER_DETAIL.
ORDER_DETAIL is the child table of ORDERS .
This is what I understand about Nested Loop:
When we join ORDER AND ORDER_DETAIL tables oracle will form a 'nested loop' in which for each order_ID in ORDER table (outer loop), oracle will look for corresponding multiple ORDER_IDs in the ORDER_DETAIL table.
Is nested loop used when the driving table (ORDER in this case) is smaller than the child table (ORDER_DETAIL) ?
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When is it ideal to use hash joins ?

Your description of a nested loop is correct.
The overall rule is that Oracle will use the plan that it calculates to be, in general, fastest. That mostly means fewest I/O's, but there are various factors that adjust its calculation - e.g. it expects index blocks to be cached, multiple reads entries in an index may reference the same block, full scans get multiple blocks per I/O. It also has CPU cost calculations, but they generally only become significant with function / package calls or domain indexes (spatial, text, ...).
Nested loop with an index will require one indexed read of the child table per outer table row, so its I/O cost is roughly twice the number of rows expected to match the where clause conditions on the outer table.
A hash join reads the of the smaller table into a hash table then matches the rows from the larger table against the hash table, so its I/O cost is the cost of a full scan of each table (unless the smaller table is too big to fit in a single in-memory hash table). Hash joins generally don't use indexes - it doesn't use the index to look up each result. It can use an index as a data source, as a narrow version of the table or a way to identify the rows satisfying the other where clause conditions.
If you are processing the whole of both tables, Oracle is likely to use a hash join, and be very fast and efficient about it.
If your where clause restricts it to a just few rows from the parent table and a few corresponding rows from the child table, and you have an index Oracle is likely to use a nested loops solution.
If the tables are very small, either plan is efficient - you may be surprised by its choice.
Don't be worry about plans with full scans and hash joins - they are usually very fast and efficient. Often bad performance comes from having to do nested loop lookups for lots of rows.

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I have seen that nested loops are better if the inner table is being indexed, because for each outer table row, we are looking for a match in the inner table. But is there any case when optimizer still goes for a nested loop even if there is no index on the inner table. That is my first question ?
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My response should really have examples but since I do not have any handy I will just say about your first question. If there is no index available from table A to table B yes it is possible a nested loop join may still be used and table B read via full table scan within a nested loop. If table B is very small and consists of only a block or two this may be relatively efficient plan. It is more likely you sould see table B full scanned and the result feed into a hash join, but I have seen the plan you mention.
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DB: 11gR1
I have gotten two conflicting answers while reading books by Don Burleson and Dan Hotka. It has to do with the CBO and nested-joins:
One says the CBO will choose the 'smaller' table as the driving table, the other states that the 'larger' table will be the driving table. And both stick by this philosophy as the preferred goal of any SQL Tuning -- that is, one states that the 'smaller' table should be the driving table. The other says the 'larger' table should be the driving table.
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user601798 wrote:
It is an over-simplistic scenario but, as I mentioned, if all other things are 'equal' -- which would include 'access time/work', then I think the small table as the driving table has the advantage.
It is not possible for +"*all* other things to be equal"+. (my emphasis).
If by +'access time/work'+ you mean the total is the same then it doesn't matter which table is first, the time/work is the same either way round.
If you want to say that the +'access time/work'+ for acquiring the first rowsource is the same for both paths, and the +'access time/work'+ for acquiring related rows from the second table is the same FOR EACH DRIVING ROW, then the total +'access time/work'+ will be difference, and it would be better to start with the smaller table. (The example by Salman Qureshi above: Re: CBO (optimizer) nest-loop join question would apply.)
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<ul>
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</ul>
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<ul>
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http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
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I can't understand what 'inner' / 'outer'
table means in nested loops join operation.
please explain the exact meaning.
maybe i do not understand the nested loops
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If I understand correctly your question. An outer table loop is where you have a table with a primary key (master table) and you want to iterate into that table which have details forign key (inner loop table) for example you have customers table each have many invoices.
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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by 4baf:
I can't understand what 'inner' / 'outer'
table means in nested loops join operation.
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When does oracle use a complete nested loop join?

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