最佳字段顺序(结论:越往后的列访问CPU开销大)
验证脚本1 (先构造出表和数据)
SET SERVEROUTPUT ON
SET ECHO ON
---构造出有25个字段的表T
DROP TABLE t;
DECLARE
l_sql VARCHAR2(32767);
BEGIN
l_sql := 'CREATE TABLE t (';
FOR i IN 1..25
LOOP
l_sql := l_sql || 'n' || i || ' NUMBER,';
END LOOP;
l_sql := l_sql || 'pad VARCHAR2(1000)) PCTFREE 10';
EXECUTE IMMEDIATE l_sql;
END;
/
----将记录还有这个表T中填充
DECLARE
l_sql VARCHAR2(32767);
BEGIN
l_sql := 'INSERT INTO t SELECT ';
FOR i IN 1..25
LOOP
l_sql := l_sql || '0,';
END LOOP;
l_sql := l_sql || 'NULL FROM dual CONNECT BY level <= 10000';
EXECUTE IMMEDIATE l_sql;
COMMIT;
END;
/
--验证脚本2(一次访问该表各字段验证)
execute dbms_stats.gather_table_stats(ownname=>user, tabname=>'t')
SELECT num_rows, blocks FROM user_tables WHERE table_name = 'T';
--以下动作观察执行速度,比较发现COUNT(*)最快,COUNT(最大列)最慢
DECLARE
l_dummy PLS_INTEGER;
l_start PLS_INTEGER;
l_stop PLS_INTEGER;
l_sql VARCHAR2(100);
BEGIN
l_start := dbms_utility.get_time;
FOR j IN 1..1000
LOOP
EXECUTE IMMEDIATE 'SELECT count(*) FROM t' INTO l_dummy;
END LOOP;
l_stop := dbms_utility.get_time;
dbms_output.put_line((l_stop-l_start)/100);
FOR i IN 1..25
LOOP
l_sql := 'SELECT count(n' || i || ') FROM t';
l_start := dbms_utility.get_time;
FOR j IN 1..1000
LOOP
EXECUTE IMMEDIATE l_sql INTO l_dummy;
END LOOP;
l_stop := dbms_utility.get_time;
dbms_output.put_line((l_stop-l_start)/100);
END LOOP;
END;
/
--结论:
--原来优化器是这么搞的:列的偏移量决定性能,列越靠后,访问的开销越大。由于count(*)的算法与列偏移量无关,所以count(*)最快。
--后面还有看图说话,看看结果输出的趋势图,就更了然了。
本文标题:count列存放的位置,越往后开销的cpu越大
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