要“强制”Oracle 使用索引范围扫描,只需使用优化器提示INDEX_RS_ASC
。例如:
CREATE TABLE mytable (a NUMBER NOT NULL, b NUMBER NOT NULL, c CHAR(10)) NOLOGGING;
INSERT /*+ APPEND */ INTO mytable(a,b,c)
SELECT level, mod(level,100)+1, 'a' FROM dual CONNECT BY level <= 1E6;
CREATE INDEX myindex_ba ON mytable(b, a);
EXECUTE dbms_stats.gather_table_stats(NULL,'mytable');
SELECT /*+ FULL(m) */ b FROM mytable m WHERE b=10; -- full table scan
SELECT /*+ INDEX_RS_ASC(m) */ b FROM mytable m WHERE b=10; -- index range scan
SELECT /*+ INDEX_FFS(m) */ b FROM mytable m WHERE b=10; -- index fast full scan
这是否会使您的查询实际上运行得更快取决于许多因素,例如索引值的选择性或表中行的物理顺序。例如,如果您将查询更改为WHERE b BETWEEN 10 AND <xxx>
,以下成本出现在我的机器上的执行计划中:
b BETWEEN 10 AND 10 20 40 80
FULL 749 750 751 752
INDEX_RS_ASC 29 325 865 1943
INDEX_FFS 597 598 599 601
如果您将查询稍微更改为不仅选择索引列b
,还有其他非索引列,成本变化很大:
b BETWEEN 10 AND 10 20 40 80
FULL 749 750 751 754
INDEX_RS_ASC 3352 40540 108215 243563
INDEX_FFS 3352 40540 108215 243563