Re: Hash join order

<pratap.deshmukh_at_gmail.com> wrote in message news:eba3c474-df59-43bc-ae13-99f3ec004ce4_at_m44g2000hsc.googlegroups.com...
> Hi,
>
> We are using Oracle 10.2.0.3.0. I have a simple query like this -
>
> select *
> from large_table, small_table
> where small_table.col = large_table.col (+)
>
> large_table has 140 million rows. Small table can have any number of
> rows.I am posting the execution plan for different scenarios where
> small table has different number of records.
>
> Case 1 - Small table has 180K records and large table has 140 million
> records. The execution plan shows that small table is accessed first
> followed by large_table and the large_table probes the hash table of
> small table
>
> ------------------------------------------------------------------------------------------------------------------------------
> | Id | Operation | Name | Rows |
> Bytes |TempSpc| Cost (%CPU)| Time | Pstart| Pstop |
> ------------------------------------------------------------------------------------------------------------------------------
> | 0 | SELECT STATEMENT | |
> 180K| 122M| | 272K (3)| 01:03:42 | | |
> |* 1 | HASH JOIN OUTER | |
> 180K| 122M| 122M| 272K (3)| 01:03:42 | | |
> | 2 | PARTITION RANGE SINGLE| |
> 180K| 120M| | 3079 (4)| 00:00:44 | 3 | 3 |
> | 3 | PARTITION LIST ALL | |
> 180K| 120M| | 3079 (4)| 00:00:44 | 1 | 25 |
> |* 4 | TABLE ACCESS FULL | SMALL_TABLE |
> 180K| 120M| | 3079 (4)| 00:00:44 | 51 | 75 |
> | 5 | INDEX FAST FULL SCAN | LARGE_TABLE_INDEX |
> 140M| 2007M| | 105K (3)| 00:24:40 | | |
> ------------------------------------------------------------------------------------------------------------------------------
>
> Case 2 - Large table has 140 million records and small table has 12
> million records. But this time the join order is reversed
>
> ------------------------------------------------------------------------------------------------------------------------------
> | Id | Operation | Name | Rows |
> Bytes |TempSpc| Cost (%CPU)| Time | Pstart| Pstop |
> ------------------------------------------------------------------------------------------------------------------------------
> | 0 | SELECT STATEMENT | |
> 12M| 8234M| | 815K (2)| 03:10:13 | | |
> |* 1 | HASH JOIN RIGHT OUTER | |
> 12M| 8234M| 3614M| 815K (2)| 03:10:13 | | |
> | 2 | INDEX FAST FULL SCAN | LARGE_TABLE_INDEX |
> 140M| 2007M| | 105K (3)| 00:24:40 | | |
> | 3 | PARTITION RANGE SINGLE| |
> 12M| 8062M| | 200K (4)| 00:46:50 | 1 | 1 |
> | 4 | PARTITION LIST ALL | |
> 12M| 8062M| | 200K (4)| 00:46:50 | 1 | 25 |
> |* 5 | TABLE ACCESS FULL | SMALL_TABLE |
> 12M| 8062M| | 200K (4)| 00:46:50 | 1 | 25 |
> ------------------------------------------------------------------------------------------------------------------------------
>
> Question is - Why is Oracle reversing the join order in Case 2 when
> ideally it should be probing the large table into the hash of the
> small table?
>
> Regards,
> Pratap
>
>

It's not the number of rows that counts, it's the total size of the hash table. According to the figures "large_table" will supply 2007M of data, and "small_table" will supply 8062M, which makes "large_table" the smaller data set ... hence the build table.

-- 
Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
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