Cardinality and the Optimizer
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The document discusses how incorrect cardinality estimates can lead to suboptimal query plans. It shows an example where statistics are gathered on a table with two indexed columns but the cardinality estimates are incorrect for queries filtering on both columns. Enabling dynamic sampling improves the estimates and chooses a better plan. The conclusion is that disabling or hinting queries is not ideal, and the goal should be to obtain correct cardinality estimates without hints so that the optimizer can choose optimal plans.
Cardinality and the Optimizer
- 1. Why do we gather statistics?
- 3. “Wrong Cardinality => Wrong Plan”
- 4. “Wrong Plan => Wrong Cardinality”
- 5. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> create table t 2 as select decode( mod(rownum,2), 0, 'N', 'Y' ) flag1, 3 decode( mod(rownum,2), 0, 'Y', 'N' ) flag2, a.* 4 from all_objects a 5 / Table created. ops$tkyte%ORA11GR2> create index t_idx on t(flag1,flag2); Index created. ops$tkyte%ORA11GR2> begin 2 dbms_stats.gather_table_stats 3 ( user, 'T', 4 method_opt=>'for all indexed columns size 254' ); 5 end; 6 / PL/SQL procedure successfully completed.
- 6. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select 'select * from t', num_rows 2 from user_tables where table_name = 'T' 3 union all 4 select 'select * from t where flag1 = "N"', num_rows/2 5 from user_tables where table_name = 'T' 6 union all 7 select 'select * from t where flag2 = "N"', num_rows/2 8 from user_tables where table_name = 'T' 9 union all 10 select 'select * from t where flag1 = "N" and flag2 = "N"', num_rows/2/2 11 from user_tables where table_name = 'T'; 'SELECT*FROMT' NUM_ROWS ------------------------------------------------- ---------- select * from t 72726 select * from t where flag1 = "N" 36363 select * from t where flag2 = "N" 36363 select * from t where flag1 = "N" and flag2 = "N" 18181.5
- 7. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> set autotrace traceonly explain ops$tkyte%ORA11GR2> select * from t where flag1='N'; -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 36499 | 3635K| 301 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T | 36499 | 3635K| 301 (1)| 00:00:04 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("FLAG1"='N')
- 8. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select * from t where flag2='N'; -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 36227 | 3608K| 301 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T | 36227 | 3608K| 301 (1)| 00:00:04 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("FLAG2"='N')
- 9. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select * from t where flag1='N' and flag2='N'; -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 18181 | 1810K| 301 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T | 18181 | 1810K| 301 (1)| 00:00:04 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("FLAG2"='N' AND "FLAG1"='N')
- 10. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select /*+ gather_plan_statistics */ * 2 from t where flag1='N' and flag2='N'; no rows selected
- 11. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------ | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.02 | 1080 | |* 1 | TABLE ACCESS FULL| T | 1 | 18181 | 0 |00:00:00.02 | 1080 | ------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(("FLAG2"='N' AND "FLAG1"='N')) 19 rows selected.
- 12. Wrong Plan => Wrong Cardinality ops$tkyte%ORA11GR2> select /*+ dynamic_sampling(t 3) */ * from t where flag1='N' and flag2='N'; ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 6 | 612 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| T | 6 | 612 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | T_IDX | 6 | | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("FLAG1"='N' AND "FLAG2"='N') Note ----- - dynamic sampling used for this statement (level=2)
- 13. Wrong Plan => Wrong Cardinality SELECT /* OPT_DYN_SAMP */ /*+ ALL_ROWS IGNORE_WHERE_CLAUSE NO_PARALLEL(SAMPLESUB) opt_param('parallel_execution_enabled', 'false') NO_PARALLEL_INDEX(SAMPLESUB) NO_SQL_TUNE */ NVL(SUM(C1),:"SYS_B_00"), NVL(SUM(C2),:"SYS_B_01"), NVL(SUM(C3),:"SYS_B_02") FROM (SELECT /*+ IGNORE_WHERE_CLAUSE NO_PARALLEL("T") FULL("T") NO_PARALLEL_INDEX("T") */ :"SYS_B_03" AS C1, CASE WHEN "T"."FLAG1"= :"SYS_B_04" AND "T"."FLAG2"=:"SYS_B_05" THEN :"SYS_B_06" ELSE :"SYS_B_07" END AS C2, CASE WHEN "T"."FLAG2"=:"SYS_B_08" AND "T"."FLAG1"=:"SYS_B_09“ THEN :"SYS_B_10" ELSE :"SYS_B_11" END AS C3 FROM "OPS$TKYTE"."T" SAMPLE BLOCK (:"SYS_B_12" , :"SYS_B_13") SEED (:"SYS_B_14") "T") SAMPLESUB
- 14. X • X • X • X • X • X
- 15. Disabling/Hinting is bad Goal is to get correct cardinality If you can hint it, you can baseline it. If you baseline it – it will use that plan, and that plan can evolve over time (if we let it)