more physical reads the blocks (in this partition)

Hi all

I have the partitioned table hash on an indexed column 'id', which is non-unique and a part of my primary key. Inside each partition, lines with the same id are located close to each other which is done by dbms_redefinition reorg using orderby_cols. The intention is to reduce the amount of physical reads as there are no requests that don't filter on the column id.

However, what I see is a large number of physical reads. The first partition has roughly 80K lines, a length of 347, block size of 8K and compression mean line... resulting in 821 blocks. And when (after flushing cache in the memory buffer and shared pool) submit a query that filters on 'id' only and results is 106 selected lines, I see to roughly 1400 physical reads.

--------------------------------------------------------------------------------------------------------------------------------
| ID | Operation | Name | Lines | Bytes | Cost (% CPU). Time | Pstart. Pstop |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | 106. 36782 | 3 (0) | 00:00:01 |
| 1. SIMPLE HASH PARTITION | 106. 36782 | 3 (0) | 00:00:01 | 1. 1.
| 2. TABLE ACCESS BY LOCAL INDEX ROWID | XXX MOVIES | 106. 36782 | 3 (0) | 00:00:01 | 1. 1.
|* 3 | INDEX RANGE SCAN | XXXXX | 1 | | 1 (0) | 00:00:01 | 1. 1.
--------------------------------------------------------------------------------------------------------------------------------

Information of predicates (identified by the operation identity card):
---------------------------------------------------

3 - access ("ID" = 49743)


Statistics
----------------------------------------------------------
22243 recursive calls
0 db block Gets
66651 compatible Gets
physical reads 1404
0 redo size
10933 bytes sent via SQL * Net to client
299 bytes received via SQL * Net from client
9 SQL * Net back and forth to and from the client
150 sorts (memory)
0 sorts (disk)
106 lines processed


I expect to see around 10-15 physical reads. And I don't understand how it can be 1400 physical reads with only 821 blocks in this partition. Can someone explain this behavior to me?

Thanks in advance,
Dirk

DRabbit wrote:
However, what I see is a large number of physical reads. The first partition has roughly 80K lines, a length of 347, block size of 8K and compression mean line... resulting in 821 blocks. And when (after flushing cache in the memory buffer and shared pool) submit a query that filters on 'id' only and results is 106 selected lines, I see to roughly 1400 physical reads.

--------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                          | Name  | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
--------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                   |       |   106 | 36782 |     3   (0)| 00:00:01 |       |       |
|   1 |  PARTITION HASH SINGLE             |       |   106 | 36782 |     3   (0)| 00:00:01 |     1 |     1 |
|   2 |   TABLE ACCESS BY LOCAL INDEX ROWID| XXX   |   106 | 36782 |     3   (0)| 00:00:01 |     1 |     1 |
|*  3 |    INDEX RANGE SCAN                | XXXXX |     1 |       |     1   (0)| 00:00:01 |     1 |     1 |
--------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

3 - access("ID"=49743)

Statistiken
----------------------------------------------------------
22243  recursive calls
0  db block gets
66651  consistent gets
1404  physical reads
0  redo size
10933  bytes sent via SQL*Net to client
299  bytes received via SQL*Net from client
9  SQL*Net roundtrips to/from client
150  sorts (memory)
0  sorts (disk)
106  rows processed

For me, the most important statistic is recursive calls. Even for a partitioned table with (for example) a few thousands of partitions, this seems very high for a simple analysis and perform call that eventually access a single partition known by index.

Test strategies - rerun the query immediately to see if you get similar results, run the query again ONLY after a buffer_cache (not shared_pool) hunt.
It is possible that more work on optimization, or calls to (for example) a pl/sql function embedded. The behavior of the repeated executions can give you a clue on this subject.
(And if none of these options can help, then activation trace and check the trace file should help).

Concerning
Jonathan Lewis

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