A SQL optimization case using order by and rownum

This is a customer AWR One of the reports TOP SQL, execution time 8.8 Hours （ There is also implementation 12.7 An error is reported for hours to exit ）

SELECT *

FROM

(

SELECT rownum num ,t.*

FROM TI_BI_CAMPN_USERORDER_TEMP t

WHERE oper_type = '0' OR oper_type = '2' OR

(oper_type = '1' AND end_date <= add_months(trunc(sysdate, 'mm'), 1))

ORDER BY oper_time ASC

) c

WHERE c.num <= : 1;

Number of table records 280M, Return to where the conditions are met and press oper_time Sort of 500 Bar record .

See above sql monitor Displayed execution plan , Some people may ask questions ：

Most records of the table are returned , Why not do a full table scan , Instead, a poor index scan is used ？

The answer is ：

Because two system parameters related to the index have been modified , Indexed COST Greatly reduced , Let the optimizer think that index walking is always better than full table scanning ：

optimizer_index_caching = 95（ The default value is 0）

optimizer_index_cost_adj = 3（ The default value is 100）

This SQL How do you optimize it ？

according to SQL monitor The information displayed and the actual number of records in the table , We can create oper_time Field to optimize ：

create index idx_name on TI_BI_CAMPN_USERORDER_TEMP(oper_time);

It is expected that after the index is created , The SQL Of The execution time should be 1~2 About seconds .

Summary ：

order by And rownum When used together , You can create predicate fields and order by Union index on field （ This example cannot be , Because there is or And asymmetric predicate conditions ）; Or when most of the data meets the conditions , establish order by Index on field （ This case ）, Avoid sorting large result sets .

Generally, it is not recommended to modify the system default parameters （ Memory parameters 、bug With the exception of ）, Because many performance tests are done under the default parameters .

Other situations ：

If this SQL Low execution frequency , You can also choose not to create an index , Use parallel and full table scanning to improve response speed , Use the following hint：

SELECT *

FROM

(

SELECT /*+ full(t) parallel(4) */rownum num ,t.*

FROM TI_BI_CAMPN_USERORDER_TEMP t

WHERE oper_type = '0' OR oper_type = '2' OR

(oper_type = '1' AND end_date <= add_months(trunc(sysdate, 'mm'), 1))

ORDER BY oper_time ASC

) c

WHERE c.num <= : 1;

If the two system parameters of the index are the default values , Among them full hint It can be removed .

The currently used parallelism is 4, The specific parallelism can be increased or decreased according to the actual needs .

Be careful ：11g The parallel writing method of does not require adding table names or aliases .10g Adding table names or aliases in is tedious and easy to omit , Throw it away ！

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