Asktom:Tom对Consitent Get的解释及分析
2011-08-31 16:27 Tracy. 阅读(317) 评论(0) 编辑 收藏 举报You Asked
Tom:
create table test( a int);
begin
for i in 1..10000 loop
insert into test values (i);
end loop;
end;
set autotrace on
select count(0) from test;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'TEST'
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
20 consistent gets
0 physical reads
0 redo size
369 bytes sent via SQL*Net to client
425 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
select * from test where a=10;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF 'TEST'
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
21 consistent gets
0 physical reads
0 redo size
360 bytes sent via SQL*Net to client
425 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
select * from test;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF 'TEST'
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
686 consistent gets
0 physical reads
0 redo size
185864 bytes sent via SQL*Net to client
74351 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10001 rows processed
question:
1.
when query count(0), what exactly did oracle do? since we don't have index, did oracle
went to every block and query the number?
if so, why consistent gets is much lower that select * from test?
both are full scan table.
2.
why select * from test have much higher consistent gets than
select * from test where a=10?
since there are no index, oracle need to go to every block to get the value, it should be
same consistent gets, although the first one return more rows, but it does't matter.
Regards,
and we said...
q1) select count(0) is just like
select count(*)
from ( select 0 from t )
/
yes, oracle went to each block to find the rows to give you a zero
q2) its a side effect of your arraysize. You must have an 8k blocksize cause I
reproduced this exactly.
We were expecting about 20 consistent gets right? Well, the default array size in
sqlplus is 15 rows / fetch. 10000/15 = 666.66666. Well, 666+20 = 686 -- whoah there --
686 is our consistent gets!
Thats what happened. When you fetched 15 rows, Oracle paused, gave you the data. When
you went back to get the next 15, it got the buffer again to resume your query.
Watch what happens with different array sizes, starting with 15:
ops$tkyte@8i> select * from test;
10000 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
686 consistent gets
0 physical reads
0 redo size
108813 bytes sent via SQL*Net to client
46265 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
ops$tkyte@8i> set arraysize 1000
ops$tkyte@8i> select * from test;
10000 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
30 consistent gets
0 physical reads
0 redo size
86266 bytes sent via SQL*Net to client
942 bytes received via SQL*Net from client
11 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
ops$tkyte@8i> set arraysize 5000
ops$tkyte@8i> select * from test;
10000 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
4 db block gets
22 consistent gets
0 physical reads
0 redo size
149793 bytes sent via SQL*Net to client
382 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
Other interesting thing to note is that as the array size gets too large -- the amount of
data transferred goes up. there is a diminishing marginal return on the array size so
don't go OVERBOARD.
followup to comment oneSet the arraysize to some constant as it will not matter for single row fetches whether
it is too big but having it too small for lots of rows (NOT just full scans -- lots of
rows) does impact you.
Followup -Excellent. May 1, 2002 - 7am Central time zone:
The count(*) returns how many rows? 1.
The array size does even begin to kick in. That statement is processed in ONE call.
Also, the count(*) may very well *never be touching the table*. It might be just using an index.
Consider:
ops$tkyte@ORA817DEV.US.ORACLE.COM> create table t as select * from all_objects;
Table created.
ops$tkyte@ORA817DEV.US.ORACLE.COM>
ops$tkyte@ORA817DEV.US.ORACLE.COM>
ops$tkyte@ORA817DEV.US.ORACLE.COM> exec show_space( 'T' )
Free Blocks.............................0
Total Blocks............................320
Total Bytes.............................2621440
Unused Blocks...........................4
Unused Bytes............................32768
Last Used Ext FileId....................7
Last Used Ext BlockId...................40969
Last Used Block.........................60
PL/SQL procedure successfully completed.
so the table consumes about 316 blocks..
ops$tkyte@ORA817DEV.US.ORACLE.COM> select * from t;
22905 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
12 db block gets
1823 consistent gets
46 physical reads
0 redo size
2704019 bytes sent via SQL*Net to client
169811 bytes received via SQL*Net from client
1528 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
22905 rows processed
22905/15 = 1527+316 = 1843 which is about our consistent gets (what we were expecting)....
ops$tkyte@ORA817DEV.US.ORACLE.COM> select count(*) from t;
Statistics
----------------------------------------------------------
0 recursive calls
12 db block gets
319 consistent gets
46 physical reads
0 redo size
369 bytes sent via SQL*Net to client
425 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
And the count(*), since it was done in a SINGLE CALL -- no arraysize -- only did a consistent get
on each block ONE TIME. Hence it did 319 consistent gets (which is about the size of the table)
Now, adding a primary key to the table:
ops$tkyte@ORA817DEV.US.ORACLE.COM> alter table t add constraint t_pk primary key(object_id);
Table altered.
ops$tkyte@ORA817DEV.US.ORACLE.COM> analyze table t compute statistics;
Table analyzed.
ops$tkyte@ORA817DEV.US.ORACLE.COM> select count(*) from t;
Statistics
----------------------------------------------------------
79 recursive calls
4 db block gets
82 consistent gets
1 physical reads
0 redo size
369 bytes sent via SQL*Net to client
425 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
we can see that the consistent gets go way down. why? because the optimizer counted the rows in
the index, not the table in this case.
db_block_gets + consistent_gets = LOGICAL IO physical_reads = PHYSICAL IO
A consistent get is a block gotten in read consistent mode (point in time mode). It MAY or MAY NOT
involve reconstruction (rolling back).
Db Block Gets are CURRENT mode gets -- blocks read "as of right now".
Some blocks are processed more then once, yes, the blocks will have more then 1 consistent read in
the process. Consider:
ops$tkyte@ORA817DEV.US.ORACLE.COM> create table t as select * from all_objects;
Table created.
ops$tkyte@ORA817DEV.US.ORACLE.COM> exec show_space( 'T')
Free Blocks.............................0
Total Blocks............................320
Total Bytes.............................2621440
Unused Blocks...........................4
Unused Bytes............................32768
Last Used Ext FileId....................7
Last Used Ext BlockId...................40969
Last Used Block.........................60
PL/SQL procedure successfully completed.
Table has 316 blocks, 22,908 rows..
ops$tkyte@ORA817DEV.US.ORACLE.COM> set autotrace traceonly statistics;
ops$tkyte@ORA817DEV.US.ORACLE.COM> set arraysize 15
ops$tkyte@ORA817DEV.US.ORACLE.COM> select * from t;
22908 rows selected.
here with an array size of 15, we expect
22908/15 + 316 = 1843 consistent mode gets. db block gets -- they were for performing the FULL
SCAN, they had nothing to do with the data itself we selected
Statistics
----------------------------------------------------------
0 recursive calls
12 db block gets
1824 consistent gets
170 physical reads
0 redo size
2704448 bytes sent via SQL*Net to client
169922 bytes received via SQL*Net from client
1529 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
22908 rows processed
ops$tkyte@ORA817DEV.US.ORACLE.COM> set arraysize 100
ops$tkyte@ORA817DEV.US.ORACLE.COM> select * from t;
22908 rows selected.
Now, with 100 as the arraysize, we expect
22908/100 + 316 = 545 consistent mode gets.
Statistics
----------------------------------------------------------
0 recursive calls
12 db block gets
546 consistent gets
180 physical reads
0 redo size
2557774 bytes sent via SQL*Net to client
25844 bytes received via SQL*Net from client
231 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
22908 rows processed
ops$tkyte@ORA817DEV.US.ORACLE.COM> set arraysize 1000
ops$tkyte@ORA817DEV.US.ORACLE.COM> select * from t;
22908 rows selected.
now, with arraysize = 1000, we expect:
22908/1000+316 = 338 consistent mode gets...
Statistics
----------------------------------------------------------
0 recursive calls
12 db block gets
342 consistent gets
222 physical reads
0 redo size
2534383 bytes sent via SQL*Net to client
2867 bytes received via SQL*Net from client
24 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
22908 rows processed
so yes, the blocks are gotten in consistent mode MORE THEN ONCE when the array fetch size is
lower then the number of rows to be retrieved in this case
This is because we'll be 1/2 way through processing a block -- have enough rows to return to the
client -- and we'll give UP that block. When they ask for the next N rows, we need to get that
halfway processed block again and pick up where we left off.
Sorry --- I've explained this beyond my best ability at this point. I've explained many many
times. This is my last try:
The table has 20 blocks in it.
You would expect 20 consistent gets on a full scan.
But, with an array size of 15 we will revisit some blocks MORE THEN ONCE.
In fact, we might revisit upto 10000/15 = 666 blocks more then ONCE.
This is because we "start" and "stop" the query in the middle of a block. (see the two followups
immediately above this).
If you full scan a table T that has N blocks and R rows and you use an array fetch size of A, we
will typically perform the following number of consistent gets:
N + R/A
We have to read N blocks (that should be obvious)
We might revisit R/A blocks more then once (since we stop in the middle)
本文来自博客园，作者：Tracy.，转载请注明原文链接：https://www.cnblogs.com/tracy/archive/2011/08/31/2160888.html
