ORACLE SQL 优化
一、只有大表才会产生性能问题
当一列出现再where条件中，并且其选择性大于20%
（选择性就是基数与总行数的比值再乘以100 num_distinct/num_rows*100）
--sql语句全自动化的优化脚本
--刷新数据库监控信息。
begin
dbms_stats.flush_database_monitoring_info;
end;
--然后我们查看 MSG_DETAILS 表有哪些列出现在 where 条件中。
select r.name owner,
o.name table_name,
c.name column_name,
equality_preds, ---等值过滤
equijoin_preds, ---等值 JOIN 比如 where a.id=b.id
nonequijoin_preds, ----不等 JOIN
range_preds, ----范围过滤次数 > >= < <= between and
like_preds, ----LIKE 过滤
null_preds, ----NULL 过滤
timestamp
from sys.col_usage$ u, sys.obj$ o, sys.col$ c, sys.user$ r
where o.obj# = u.obj#
and c.obj# = u.obj#
and c.col# = u.intcol#
and r.name = 'YYSMS'
and o.name = 'MSG_DETAILS';
--接下来我们查询出选择性大于等于 20%的列。
select a.owner,
a.table_name,
a.column_name,
round(a.num_distinct / b.num_rows * 100, 2) selectivity
from dba_tab_col_statistics a, dba_tables b
where a.owner = b.owner
and a.table_name = b.table_name
and a.owner = 'YYSMS'
and a.table_name ='MSG_DETAILS'
and a.num_distinct / b.num_rows >= 0.2;
--最后，确保这些列没有创建索引。
select table_owner, table_name, column_name, index_name
from dba_ind_columns
where table_owner = 'YYSMS'
and table_name = 'MSG_DETAILS';
--把上面的脚本组合起来，我们就可以得到全自动的优化脚本了
select owner,
column_name,
num_rows,
Cardinality,
selectivity,
'Need index' as notice
from (select b.owner,
a.column_name,
b.num_rows,
a.num_distinct Cardinality,
round(a.num_distinct / b.num_rows * 100, 2) selectivity
from dba_tab_col_statistics a, dba_tables b
where a.owner = b.owner
and a.table_name = b.table_name
and a.owner = 'YYSMS'
and a.table_name ='MSG_DETAILS')
where selectivity >= 20
and column_name not in (select column_name
from dba_ind_columns
where table_owner = 'YYSMS'
and table_name ='MSG_DETAILS')
and column_name in
(select c.name
from sys.col_usage$ u, sys.obj$ o, sys.col$ c, sys.user$ r
where o.obj# = u.obj#
and c.obj# = u.obj#
and c.col# = u.intcol#
and r.name = 'YYSMS'
and o.name = 'MSG_DETAILS');
二、如果某个列基数很低，则说明该列的数据分布就非常不均衡，由于该列分布不均衡，所以可能走索引也可能走全表扫描，这个时候就很容易走错执行计划。此时应该收集直方图统计信息，否则基于成本的优化器CBO会认为该列数据分布式均衡的。
对某一列收集直方图统计信息
--现在我们对 cc列收集直方图。
BEGIN
DBMS_STATS.GATHER_TABLE_STATS(ownname => 'YYSMS',
tabname => 'MSG_DETAILS',
estimate_percent => 100,
method_opt => 'for columns cc size skewonly',
no_invalidate => FALSE,
degree => 1,
cascade => TRUE);
END;
/
PL/SQL procedure successfully completed.
-- 查看一下 cc 列的直方图信息
select a.column_name,
b.num_rows,
a.num_distinct Cardinality,
round(a.num_distinct / b.num_rows * 100, 2) selectivity,
a.histogram,
a.num_buckets
from dba_tab_col_statistics a, dba_tables b
where a.owner = b.owner
and a.table_name = b.table_name
and a.owner = 'YYSMS'
and a.table_name = 'MSG_DETAILS';
--抓出必须创建直方图的列（大家可以对该脚本进行适当修改，以便用于生产环境）。
select a.owner,
a.table_name,
a.column_name,
b.num_rows,
a.num_distinct,
trunc(num_distinct / num_rows * 100,2) selectivity,
'Need Gather Histogram' notice
from dba_tab_col_statistics a, dba_tables b
where a.owner = 'YYSMS'
and a.table_name = 'MSG_DETAILS'
and a.owner = b.owner
and a.table_name = b.table_name
and num_distinct / num_rows<0.01
and (a.owner, a.table_name, a.column_name) in
(select r.name owner, o.name table_name, c.name column_name
from sys.col_usage$ u, sys.obj$ o, sys.col$ c, sys.user$ r
where o.obj# = u.obj#
and c.obj# = u.obj#
and c.col# = u.intcol#
and r.name = 'YYSMS'
and o.name = 'MSG_DETAILS')
and a.histogram ='NONE';