SQL> list
1 SELECT *
2 FROM dept, emp
3* WHERE emp.deptno = dept.deptno
SQL> set autotrace traceonly /*traceonly 可以不显示执行结果*/
SQL> /
14 rows selected.
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (FULL) OF 'EMP'
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
Statistics
----------------------------------------------------------
0 recursive calls
2 db block gets
30 consistent gets
0 physical reads
0 redo size
2598 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
14 rows processed
通过以上分析,可以得出实际的执行步骤是:
1. TABLE ACCESS (FULL) OF 'EMP'
2. INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
3. TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4. NESTED LOOPS (JOINING 1 AND 3)
怎么样才可以看出执行步骤是这样的呀?
1 SELECT *
2 FROM dept, emp
3* WHERE emp.deptno = dept.deptno
SQL> set autotrace traceonly /*traceonly 可以不显示执行结果*/
SQL> /
14 rows selected.
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (FULL) OF 'EMP'
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
Statistics
----------------------------------------------------------
0 recursive calls
2 db block gets
30 consistent gets
0 physical reads
0 redo size
2598 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
14 rows processed
通过以上分析,可以得出实际的执行步骤是:
1. TABLE ACCESS (FULL) OF 'EMP'
2. INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
3. TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4. NESTED LOOPS (JOINING 1 AND 3)
怎么样才可以看出执行步骤是这样的呀?
1 0 NESTED LOOPS --nested
2 1 TABLE ACCESS (FULL) OF 'EMP' --3:full scan
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT' --2:rowid scan
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE) --1:index scan
1 0 NESTED LOOPS --这是第四步,nested
2 1 TABLE ACCESS (FULL) OF 'EMP' --这是第三步,全表扫描
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT' --这是第二步,通过ROWID访问表
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE) --这是第一步,先index scan
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (FULL) OF 'EMP'
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)但不知道为什么书上写的顺序是这样的:“
通过以上分析,可以得出实际的执行步骤是:
1. TABLE ACCESS (FULL) OF 'EMP'
2. INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
3. TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4. NESTED LOOPS (JOINING 1 AND 3) ”
所以我才不明白!难道是人家写错了,但是写的错的话,这样的顺序也太离普了吧!
TABLE ACCESS (FULL) OF 'EMP'
和
INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
属于最先执行的操作