Tuesday, July 31, 2012

What extended stats do I have on my database?

I've been starting to work with Extended statistics to help the optimizer find the best plan. This is a great feature that is outlined by @sqlmaria (Maria Coogan) here.

But once you create extended statistics, how do you know what is there ?  I wrote this query to find out what function based indexes, what extended statistics, and what their definition are. 

Here is my script.

column table_owner alias "owner" format a15
column table_name alias  "Table Name" format  a30
column function_index alias  "F Index" format  a8
column Index_name  alias  "Index Name"  format a30
column data_default alias  "Definition"  format a50
set pagesize 1000
select table_owner,
         table_name,
        nvl2(index_name,'YES','NO') function_index,
        index_name,
        data_default
        from
        (
select owner table_owner,table_name,
(select distinct index_name from dba_ind_columns b where a.column_name=b.column_name and a.owner=b.index_owner and a.table_name=b.table_name) index_name
,data_default
-- ,     DBMS_LOB.SUBSTR( to_lob(data_default),100,1)
 from dba_tab_cols a
  where virtual_column='YES' and hidden_column='YES'  and (owner not in ('SYS','WMSYS','XDB','SYSMAN','MDSYS','EXFSYS','PR_MDS') and owner not like 'APEX_%')
  )
order by table_owner,table_name;


and this is what the output looks like..

TABLE_OWNER     TABLE_NAME                     FUNCTION INDEX_NAME                     DATA_DEFINITION
--------------- ------------------------------ -------- ------------------------------ --------------------------------------------------
BGRENN          TAB_SCHR_PERD                      NO                                  COALESCE("COL1","COL2")
BGRENN          TAB2                              YES   IDX_TAB2                       "COL1"||' '||"COL2"
BGRENN          TAB3                               NO                                  COALESCE("COL1","COL2")
BGRENN          TAB4                              YES   IDX_TAB4                       COALESCE("COL1","COL2",0)
BGRENN          TAB4                              YES   IDX_TAB4                       COALESCE("COL3",0)
BGRENN          TAB5                              YES   IDX_TAB5                       COALESCE("COL1","COL2",0)
BGRENN          TAB6                              YES   IDX_TAB6                       NVL("COL1",(-1))
BGRENN          TAB6                              YES   IDX_TAB6                       NVL("COL2",(-1))
BGRENN          TAB6                              YES   IDX_TAB6                       NVL("COL3",(-1))
BGRENN          TAB6                              YES   IDX_TAB6                       NVL("COL4",'x')
BGRENN          TAB7                              YES   IDX_COMPOSITE                  "COL1"
BGRENN          TAB7                              YES   IDX_COMPOSITE                  "COL3"

Notice the Function colunmn. This is a "YES" or "NO" depending on if this is a function based index, or just extended statistics.

This should help tell where your extended statistics are in your database.

Sunday, July 15, 2012

Exadata tips

I wanted to write up an Exadata tip that I learned.

Background :  I wanted to do a simple "select count(1) from mytable".  mytable has a primary key on it.  The count seemed to be taking a long time for an Exadata.

First the "select count(1) from mytable". You can see that it uses an index storage fast full scan.  The top wait event is "cell multiblock physical read".  The query does  2 Million Disk reads in 3 Minutes 28 seconds.

But this seems slow...



select count(1)
from
     MYTABLE MYTABLE

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2     56.46     208.27    2030378    2031797         20           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4     56.46     208.27    2030378    2031797         20           1

Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: SYS
Number of plan statistics captured: 1

Rows (1st) Rows (avg) Rows (max)  Row Source Operation
---------- ---------- ----------  ---------------------------------------------------
         1          1          1  SORT AGGREGATE (cr=2031797 pr=2030378 pw=0 time=208276577 us)
 592848893  592848893  592848893   INDEX STORAGE FAST FULL SCAN PK_MYTABLE (cr=2031797 pr=2030378 pw=0 time=245927651 us cost=523897 size=0 card=572441788)(object id 312310)


Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  library cache lock                              4        0.00          0.00
  Disk file operations I/O                       42        0.00          0.00
  library cache pin                               2        0.00          0.00
  SQL*Net message to client                       2        0.00          0.00
  cell single block physical read                15        0.00          0.00
  cell list of blocks physical read               2        0.00          0.00
  cell multiblock physical read               15959        0.26        152.20
  latch: object queue header operation            1        0.00          0.00
  SQL*Net message from client                     2       10.38         10.38
********************************************************************************



Next I did a FTS.

"select /*+full(MYTABLE) */ count(1) from  MYTABLE MYTABLE ;

You can see this did 2.2 Million disk reads (more than the index scan), but the wait event is sql_net. With the "cell smart table scan", there were very few waits, and the wait was much shorter.



select /*+full(MYTABLE) */ count(1)
from
     MYTABLE MYTABLE  

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2     65.11      66.48    2224642    2225028         21           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4     65.11      66.48    2224642    2225028         21           1

Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: SYS
Number of plan statistics captured: 1

Rows (1st) Rows (avg) Rows (max)  Row Source Operation
---------- ---------- ----------  ---------------------------------------------------
         1          1          1  SORT AGGREGATE (cr=2225028 pr=2224642 pw=0 time=66486729 us)
 592848893  592848893  592848893   PARTITION RANGE ALL PARTITION: 1 33 (cr=2225028 pr=2224642 pw=0 time=140325566 us cost=533066 size=0 card=572441788)
 592848893  592848893  592848893    TABLE ACCESS STORAGE FULL MYTABLE PARTITION: 1 33 (cr=2225028 pr=2224642 pw=0 time=54479242 us cost=533066 size=0 card=572441788)


Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  SQL*Net message to client                       2        0.00          0.00
  Disk file operations I/O                      129        0.00          0.00
  gc current block 2-way                         65        0.00          0.00
  enq: KO - fast object checkpoint              101        0.00          0.02
  reliable message                               33        0.09          0.30
  gc current block 3-way                         13        0.00          0.00
  cell smart table scan                        1403        0.03          1.01
  gc cr block 3-way                               1        0.00          0.00
  gc current grant busy                          18        0.00          0.00
  gc cr block 2-way                              17        0.00          0.00
  gc cr multi block request                       5        0.00          0.00
  cell single block physical read                11        0.00          0.00
  cell list of blocks physical read               2        0.00          0.00
  gc cr grant 2-way                               3        0.00          0.00
  SQL*Net message from client                     2        9.62          9.62






Bottom line, if you want to a count on a table use the "FULL" hint.  The exadata is built for table scans, and this example shows that.

It also should make you rethink when to use indexes for an application, you see they can hurt you in some cases.

Friday, July 6, 2012

What happened to my sql (sql_id) ?


While finishing up a few things, I ran across a query that wasn't playing nicely. It had 4 different plans over the course of the last couple of days, and I wanted to see what happend.. I came up with the nifty query below.  If you plug in a sql_id, it will go through the AWR history, and return (ordered by date last executed), the plans grouped by plan_hash_value. Within each plan_hash_value it will give you the objects in the plan, and when they were last analyzed.  By using this you should see what plans are good, when they were last executed, and if anything was analyzed to change the plan.


set linesize 160
set pagesize 1000
break on plan_hash_value skip 1 nodup  on last_executed skip 1 nodup  on avg_exec_time skip 1
select object_owner ||'.'|| object_name object_name,
object_type,
a.plan_hash_value,
case object_type
  when 'INDEX' then (select last_analyzed from dba_indexes b where owner=object_owner and index_name=object_name)
  when 'TABLE'  then (select last_analyzed from dba_tables b where owner=object_owner and table_name=object_name)
 else null 
end last_analyzed,
 to_char((select max(end_interval_time) from dba_hist_snapshot b,
                                           dba_hist_sqlstat c 
                            where c.sql_id=a.sql_id and 
                                  c.plan_hash_value=a.plan_hash_value and 
                              b.snap_id=c.snap_id),'mm/dd/yy hh24:mi') last_Executed,
to_char((select sum(elapsed_time_delta)/sum(executions_delta) from dba_hist_sqlstat d where d.sql_id=a.sql_id and d.plan_hash_value=a.plan_hash_value)/1024/1024,'999.99') avg_exec_time
 from DBA_HIST_SQL_PLAN  a
where a.SQL_ID='gbug7dg8adhgh'
 and object_type in ('INDEX','TABLE')
order by last_executed desc ,a.plan_hash_value , last_analyzed desc;


Here is an example of the output


              OBJECT_NAME                                 OBJECT_TYPE          PLAN_HASH_VALUE LAST_ANALYZED       LAST_EXECUTED  AVG_EXE
-------------------------------------------------------------- -------------------- --------------- ------------------- -------------- -------
MY_SCHEMA.SNP_CDC_SUBS                                     TABLE                     2518369181 2012-07-06 09:25:15 07/06/12 10:00  791.37
MY_SCHEMA.SNP_CDC_SUBS                                     TABLE                                2012-07-06 09:25:15
MY_SCHEMA.D$TAB_REG                                        TABLE                                2012-07-06 09:25:06
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
ERD.DIM_TABS_COMP_PLCY_AGMT                                TABLE                                2012-07-06 00:39:33
MY_SCHEMA.TAB_COMP_PLCY_AGMT                               TABLE                                2012-05-25 09:20:39
MY_SCHEMA.IDX_WCPA_AGMT_ID                                 INDEX                                2012-05-25 09:20:39
MY_SCHEMA.TAB_COMP_PLCY_ST_CLSF_VT                         TABLE                                2012-05-15 18:49:43
MY_SCHEMA.TAB_COMP_PLCY_ST_CLSF                            TABLE                                2012-05-15 18:49:43
MY_SCHEMA.WPTD_COMP_PER_TAX                                TABLE                                2012-05-15 18:39:30
MY_SCHEMA.TAB_REG                                          TABLE                                2012-05-15 18:31:18
MY_SCHEMA.CO_TAB                                           TABLE                                2012-05-15 18:27:50
MY_SCHEMA.TAB_PAYR                                         TABLE                                2012-05-15 18:26:47
MY_SCHEMA.AGMT_REG                                         TABLE                                2012-05-15 18:21:09



MY_SCHEMA.SNP_CDC_SUBS                                     TABLE                     1903861587 2012-07-06 09:25:15 07/06/12 09:00  882.94
MY_SCHEMA.SNP_CDC_SUBS                                     TABLE                                2012-07-06 09:25:15
MY_SCHEMA.D$TAB_REG                                        TABLE                                2012-07-06 09:25:06
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
MY_SCHEMA.J$TAB_REG                                        TABLE                                2012-07-06 09:24:50
ERD.DIM_TABS_COMP_PLCY_AGMT                                TABLE                                2012-07-06 00:39:33
MY_SCHEMA.TAB_COMP_PLCY_AGMT                               TABLE                                2012-05-25 09:20:39
MY_SCHEMA.IDX_WCPA_AGMT_ID                                 INDEX                                2012-05-25 09:20:39
MY_SCHEMA.TAB_COMP_PLCY_ST_CLSF_VT                         TABLE                                2012-05-15 18:49:43
MY_SCHEMA.TAB_COMP_PLCY_ST_CLSF                            TABLE                                2012-05-15 18:49:43
MY_SCHEMA.WPTD_COMP_PER_TAX                                TABLE                                2012-05-15 18:39:30
MY_SCHEMA.TAB_REG                                          TABLE                                2012-05-15 18:31:18
MY_SCHEMA.CO_TAB                                           TABLE                                2012-05-15 18:27:50
MY_SCHEMA.TAB_PAYR                                         TABLE                                2012-05-15 18:26:47
MY_SCHEMA.AGMT_REG                                         TABLE                                2012-05-15 18:21:09