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Oracle 12.1.0.2新特性 Approximate Count Distinct
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Oracle 12.1.0.2新特性 Approximate Count Distinct
[oracle@PD009 ~]$ grep -i approx_count oracle.str settings for approx_count_distinct optimizations qkaGBPushdown: estimated memory without GPD = groupSize (%.2f) * aclsum (%u) = %.2f; estimated memory with GPD = optHllEntry (%u) * # of approx_count_distinct (%d) * parallelDegree (%.0f) = %.2f APPROX_COUNT_DISTINCT APPROX_COUNT_DISTINCT APPROX_COUNT_DISTINCT _approx_cnt_distinct_gby_pushdown = choose _approx_cnt_distinct_optimization = 0 alter system flush shared_pool; alter session set events '10053 trace name context forever ,level 1'; select count( distinct prod_id) from sales_history where amount_sold>1; select approx_count_distinct(prod_id) from sales_history where amount_sold>1; oradebug setmypid; oradebug tracefile_name; /s01/diag/rdbms/pdprod/PDPROD/trace/PDPROD_ora_4086.trc sql= select count( distinct prod_id) from sales_history where amount_sold>1 ----- Explain Plan Dump ----- ----- Plan Table ----- ============ Plan Table ============ ---------------------------------------------+-----------------------------------+ | Id | Operation | Name | Rows | Bytes | Cost | Time | ---------------------------------------------+-----------------------------------+ | 0 | SELECT STATEMENT | | | | 4912 | | | 1 | SORT AGGREGATE | | 1 | 13 | | | | 2 | VIEW | VW_DAG_0 | 72 | 936 | 4912 | 00:00:59 | | 3 | HASH GROUP BY | | 72 | 648 | 4912 | 00:00:59 | | 4 | TABLE ACCESS FULL | SALES_HISTORY| 3589K | 32M | 4820 | 00:00:58 | ---------------------------------------------+-----------------------------------+ sql= select approx_count_distinct(prod_id) from sales_history where amount_sold>1 ----- Explain Plan Dump ----- ----- Plan Table ----- ============ Plan Table ============ ----------------------------------------------+-----------------------------------+ | Id | Operation | Name | Rows | Bytes | Cost | Time | ----------------------------------------------+-----------------------------------+ | 0 | SELECT STATEMENT | | | | 4820 | | | 1 | SORT AGGREGATE APPROX | | 1 | 9 | | | | 2 | TABLE ACCESS FULL | SALES_HISTORY| 3589K | 32M | 4820 | 00:00:58 | ----------------------------------------------+-----------------------------------+ Predicate Information:
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