简单来看,hash join的代价主要在于建立hash表、扫描M表、join条件连接和filter过滤,对于S表和M表都是只需要扫描一次即可,filter过滤是指t1.c2>t2.c2这样的条件过滤,对于t1.c1>1这样只涉及单表的条件会被下压,在做连接之前就被过滤了。 优化器处理过后,会生成一颗执行计划树,真正的实现过程根据执行计划的流程操作...
When a view cannot be merged into the main query you will often see a projection view operation. This indicates that the 'view' will be selected from directly as opposed to being broken down into joins on the base tables. A number of constructs make a view non mergeable. Inline views ar...
所以不管是用F5方式还是set statistics_level=ALL方式,都有Operation参数,Operation表示sql执行过程,查看怎么执行的,有两个规则: 根据Operation缩进判断,缩进最多的最先执行; Operation缩进相同时,最上面的是最先执行的; 如图执行计划,根据规则,可以得出执行顺序:INDEX UNIQUE SCAN->TABLE ACCESS BY INDEX ROWID->INDEX...
[SQL] SELECT LPAD(' ', LEVEL-1) || OPERATION || ' (' || OPTIONS || ')' "Operation", OBJECT_NAME "Object", OPTIMIZER "Optimizer", COST "Cost", CARDINALITY "Cardinality", BYTES "Bytes", PARTITION_START "Partition Start", PARTITION_ID "Partition ID" , ACCESS_PREDICATES "Access Predi...
図 22:結合順序を強調表示した計画の出力例 複雑な SQL 文では,Operation 列で表のインデントを見ても,結合順序を特定するのは容易でない 場合があります.そのような場合は,DBMS_XPLAN プロシージャで FORMAT パラメータを使用し て,結合順序を含む計画の概要情報を表示する方が容易かもしれません....
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | | 0 | SELECT STATEMENT | | 13 | 988 | 6 (17)| 00:00:01 | | 1 | MERGE JOIN | | 13 | 988 | 6 (17)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 532 | 2 (0)| 00:00:01 | ...
scott@ORCL>create table t1 nologgingasselect*from t;scott@ORCL>exec dbms_stats.gather_table_stats('SCOTT','T1');--使用explain plan加载创建索引的执行计划 scott@ORCL>explain plansetstatement_id='IDX'forcreate index i_t1 ont1(object_id);Explained.scott@ORCL>@Get_Plan ...
explain planfordeletefrom t_jingyu;Explained.SQL>select*fromtable(dbms_xplan.display);PLAN_TABLE_OUTPUT---Plan hash value:1368587462---|Id|Operation|Name|Rows|Cost(%CPU)|Time|---
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --- | 0 | SELECT STATEMENT | | 8168 | 16336 | 29 (0)| 00:00:01 | | 1 | COLLECTION ITERATOR PICKLER FETCH| DISPLAY | 8168 | 16336 | 29 (0)| 00:00:01 | ---...
. Join order – The order in which the tables are joined to each other. Partition pruning – Are only the necessary partitions being accessed to answer the query? Parallel Execution – In case of parallel execution, is each operation in the plan being conducted in parallel?