Tagged: partial partition-wise join

Bloom Filters

The other day I was reading the 11g Database VLDB and Partitioning Guide and came across the below execution plan for a partial partition-wise join between sales and customers.

---------------------------------------------------------------------------
| Id  | Operation                             | Name      | Pstart| Pstop |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT                      |           |       |       |
|   1 |  PX COORDINATOR                       |           |       |       |
|   2 |   PX SEND QC (RANDOM)                 | :TQ10002  |       |       |
|*  3 |    FILTER                             |           |       |       |
|   4 |     HASH GROUP BY                     |           |       |       |
|   5 |      PX RECEIVE                       |           |       |       |
|   6 |       PX SEND HASH                    | :TQ10001  |       |       |
|   7 |        HASH GROUP BY                  |           |       |       |
|*  8 |         HASH JOIN                     |           |       |       |
|   9 |          PART JOIN FILTER CREATE      | :BF0000   |       |       |
|  10 |           PX RECEIVE                  |           |       |       |
|  11 |            PX SEND PARTITION (KEY)    | :TQ10000  |       |       |
|  12 |             PX BLOCK ITERATOR         |           |       |       |
|  13 |              TABLE ACCESS FULL        | CUSTOMERS |       |       |
|  14 |          PX PARTITION HASH JOIN-FILTER|           |:BF0000|:BF0000|
|* 15 |           TABLE ACCESS FULL           | SALES     |:BF0000|:BF0000|
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - filter(COUNT(SYS_OP_CSR(SYS_OP_MSR(COUNT(*)),0))>100)
   8 - access("S"."CUST_ID"="C"."CUST_ID")
  15 - filter("S"."TIME_ID"=TO_DATE(' 1999-07-01 00:00:00',
                                     'syyyy-mm-dd hh24:mi:ss'))

At first, it may seem just like another parallel execution plan, but if you look again you may notice that the Pstart and Pstop values are :BF0000 (id 15) and :BF0000 also appears on as part of the PART JOIN FILTER CREATE (id 9). So what exactly is :BF0000? This is a bloom filter being applied as part of the DFO (data flow operator).

It’s not necessary to completely understand bloom filters to take advantage of them, but the next time you see :BF0000 in your parallel execution plan, you will be able to recognize what it is.