[20180408]那些函数索引适合字段的查询.txt
[20180408]那些函数索引适合字段的查询.txt
--//一般不主张建立函数索引,往往是开发的无知,使用trunc等函数,实际上一些函数也可以用于字段的查询.
--//以前零碎的写过一些,放假看了https://blog.pythian.com/tackling-time-troubles-use-dates-correctly-oracle/.
--//自己也做一些总结:
1.环境:
SCOTT@test01p> @ ver1
PORT_STRING VERSION BANNER CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0 12.1.0.1.0 Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0
2.trunc函数:
--//trunc函数是开发最常使用的函数,实际上还是一句话无知,开发根本不知道这样建立查询条件导致普通索引无效,必须建立函数索引.
--//但是建立trunc函数索引却可以使用在普通日期字段的查询.
SCOTT@test01p> create index if_emp_hiredate on emp(trunc(hiredate));
Index created.
SCOTT@test01p> select * from emp where hiredate=to_date('1980/12/17','yyyy-mm-dd');
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 1ct8dum3uyy7u, child number 0
-------------------------------------
select * from emp where hiredate=to_date('1980/12/17','yyyy-mm-dd')
Plan hash value: 4059437819
--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP | 1 | 38 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IF_EMP_HIREDATE | 1 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / EMP@SEL$1
2 - SEL$1 / EMP@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("HIREDATE"=TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss'))
2 - access("EMP"."SYS_NC00010$"=TRUNC(TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss')))
--//顺便看看范围查询是否有效:
SCOTT@test01p> select * from emp where hiredate between to_date('1980/12/17','yyyy-mm-dd') and to_date('1980/12/18','yyyy-mm-dd');
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID a00watu79k28f, child number 0
-------------------------------------
select * from emp where hiredate between
to_date('1980/12/17','yyyy-mm-dd') and
to_date('1980/12/18','yyyy-mm-dd')
Plan hash value: 4059437819
--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| EMP | 1 | 38 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IF_EMP_HIREDATE | 1 | | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / EMP@SEL$1
2 - SEL$1 / EMP@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("HIREDATE"<=TO_DATE(' 1980-12-18 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND
"HIREDATE">=TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss')))
2 - access("EMP"."SYS_NC00010$">=TRUNC(TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd
hh24:mi:ss')) AND "EMP"."SYS_NC00010$"<=TRUNC(TO_DATE(' 1980-12-18 00:00:00', 'syyyy-mm-dd
hh24:mi:ss')))
--//一样有效.补充11g下执行计划:
SCOTT@book> @ &r/ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
SCOTT@book> select * from emp where hiredate between to_date('1980/12/16 09:00:00','yyyy-mm-dd hh24:mi:ss') and to_date('1980/12/18 10:00:00','yyyy-mm-dd hh24:mi:ss');
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20
--//执行计划:
SQL_ID f532a3utw8dfh, child number 0
-------------------------------------
select * from emp where hiredate between to_date('1980/12/16
09:00:00','yyyy-mm-dd hh24:mi:ss') and to_date('1980/12/18
10:00:00','yyyy-mm-dd hh24:mi:ss')
Plan hash value: 3187737602
------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
|* 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 46 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IF_EMP_HIREDATE | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / EMP@SEL$1
2 - SEL$1 / EMP@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("HIREDATE"<=TO_DATE(' 1980-12-18 10:00:00', 'syyyy-mm-dd hh24:mi:ss')
AND "HIREDATE">=TO_DATE(' 1980-12-16 09:00:00', 'syyyy-mm-dd hh24:mi:ss')))
2 - access("EMP"."SYS_NC00009$">=TRUNC(TO_DATE(' 1980-12-16 09:00:00', 'syyyy-mm-dd
hh24:mi:ss')) AND "EMP"."SYS_NC00009$"<=TRUNC(TO_DATE(' 1980-12-18 10:00:00',
'syyyy-mm-dd hh24:mi:ss')))
3.substr函数:
--//这个函数非常特殊,你建立的索引必须是substr(col,1,N)的格式,必须是从1开始截取,否则普通字段查询不会使用.这个很好理解.
--//看standard_hash函数后面的例子.
4.standard_hash函数:
--//缺点这个函数12c下才有.以前的测试,重复贴出,不想写了.
--//链接:http://blog.itpub.net/267265/viewspace-772856/
--//到目前为之,我仅仅知道这3个函数可以在普通字段查询时使用.
--//trunc,substr(Col,1,N)还合适范围查询.standard_hash(12c才有的函数)仅仅适合等值查询.
[20130916]12c Indexing Extended Data Types and index.txt
http://richardfoote.wordpress.com/2013/09/12/12c-indexing-extended-data-types-part-i-a-big-hurt/
--//参考以上链接,做一些测试:
1.测试环境:
SCOTT@test01p> @ver
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0
SCOTT@test01p> create table bowie (id number, text varchar2(32000));
Table created.
SCOTT@test01p> create index bowie_text_i on bowie(text);
create index bowie_text_i on bowie(text)
*
ERROR at line 1:
ORA-01450: maximum key length (6398) exceeded
--//超长字段无法在上建议索引。
--//12c提供standard_hash函数,可以实现其上建立函数索引。
2.插入一些数据,便于测试:
SCOTT@test01p> insert into bowie (id, text) values (1, lpad('a',1110,'a'));
1 row created.
SCOTT@test01p> commit ;
Commit complete.
SCOTT@test01p> select length(text) from bowie;
LENGTH(TEXT)
------------
1110
SCOTT@test01p> insert into bowie (id, text) select 2, text||text||text||text||text||text||text||text||text||text from bowie;
1 row created.
SCOTT@test01p> commit ;
Commit complete.
SCOTT@test01p> select length(text) from bowie;
LENGTH(TEXT)
------------
1110
11100
SCOTT@test01p> insert into bowie (id, text) select rownum+2, to_char(rownum)||'BOWIE' from dual connect by level<=99998;
99998 rows created.
SCOTT@test01p> commit ;
Commit complete.
SCOTT@test01p> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'BOWIE', method_opt=>'FOR ALL COLUMNS SIZE 1');
PL/SQL procedure successfully completed.
3.建立函数索引:
SCOTT@test01p> create index bowie_hash_text_i on bowie(standard_hash(text));
Index created.
SCOTT@test01p> select index_name, num_rows, leaf_blocks from dba_indexes where index_name = 'BOWIE_HASH_TEXT_I';
INDEX_NAME NUM_ROWS LEAF_BLOCKS
------------------ ---------- -----------
BOWIE_HASH_TEXT_I 100000 447
4.查询看看情况:
SCOTT@test01p> column text format a100
SCOTT@test01p> select * from bowie where text = '42BOWIE';
ID TEXT
---------- ---------------------------------------------------
44 42BOWIE
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 3uz6tby2rv7bh, child number 1
-------------------------------------
select * from bowie where text = '42BOWIE'
Plan hash value: 1900956348
---------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 3 (100)|
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE | 1 | 3 (0)|
|* 2 | INDEX RANGE SCAN | BOWIE_HASH_TEXT_I | 1 | 2 (0)|
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")='42BOWIE'))
2 - access("BOWIE"."SYS_NC00003$"=HEXTORAW('A2C98939EDB479BC3EB0CDC560DDCD15
75D47F62'))
--可以发现可以使用这个函数索引。
5.但是这种情况存在一些限制,做like 或者between时,不能使用该函数索引:
SCOTT@test01p> select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%';
...
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------
SQL_ID 01fn3bq946un9, child number 0
-------------------------------------
select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%'
Plan hash value: 1845943507
---------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------
| 0 | SELECT STATEMENT | | | 208 (100)|
|* 1 | TABLE ACCESS FULL| BOWIE | 1 | 208 (2)|
---------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")
LIKE 'aaaaaaaaaaaaaaaaaaaaaa%'))
SCOTT@test01p> select * from bowie where text between '4299BOWIE' and '42BOWIE';
ID TEXT
---------- ---------------------------------------------------------------------
44 42BOWIE
431 429BOWIE
4301 4299BOWIE
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 1uk9ud7fq8fdx, child number 0
-------------------------------------
select * from bowie where text between '4299BOWIE' and '42BOWIE'
Plan hash value: 1845943507
---------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------
| 0 | SELECT STATEMENT | | | 208 (100)|
|* 1 | TABLE ACCESS FULL| BOWIE | 2 | 208 (2)|
---------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")<='42BOWIE' AND
INTERNAL_FUNCTION("TEXT")>='4299BOWIE'))
SCOTT@test01p> select * from bowie where text > 'zzz';
no rows selected
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
---------------------------------------------------------------
SQL_ID 39wfprrkz66td, child number 0
-------------------------------------
select * from bowie where text > 'zzz'
Plan hash value: 1845943507
---------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------
| 0 | SELECT STATEMENT | | | 208 (100)|
|* 1 | TABLE ACCESS FULL| BOWIE | 1 | 208 (2)|
---------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")>'zzz'))
6.很明显,无法在该列上建议唯一约束。
SCOTT@test01p> alter table bowie add constraint bowie_text_unq unique (text);
alter table bowie add constraint bowie_text_unq unique (text)
*
ERROR at line 1:
ORA-01450: maximum key length (6398) exceeded
--//建立以上约束,需要在该列上建立索引,超长无法建立。同样可以变通的方法建立:
SCOTT@test01p> drop index bowie_hash_text_i;
Index dropped.
SCOTT@test01p> alter table bowie add (text_hash as (standard_hash(text)));
Table altered.
SCOTT@test01p> alter table bowie add constraint bowie_text_unq unique (text_hash);
Table altered.
SCOTT@test01p> insert into bowie (id, text) values (1000001, '42BOWIE');
insert into bowie (id, text) values (1000001, '42BOWIE')
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.BOWIE_TEXT_UNQ) violated
--再重复以上查询:
SCOTT@test01p> select * from bowie where text = '42BOWIE';
ID TEXT TEXT_HASH
---------- -------- ----------------------------------------
44 42BOWIE A2C98939EDB479BC3EB0CDC560DDCD1575D47F62
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 3uz6tby2rv7bh, child number 1
-------------------------------------
select * from bowie where text = '42BOWIE'
Plan hash value: 2691947611
----------------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 2 (100)|
|* 1 | TABLE ACCESS BY INDEX ROWID| BOWIE | 1 | 2 (0)|
|* 2 | INDEX UNIQUE SCAN | BOWIE_TEXT_UNQ | 1 | 1 (0)|
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")='42BOWIE'))
2 - access("BOWIE"."TEXT_HASH"=HEXTORAW('A2C98939EDB479BC3EB0CDC560DD
CD1575D47F62'))
--但是如果做范围查询,结果如何应该同上是选择全表扫描。
SCOTT@test01p> select * from bowie where text between '429BOWIE' and '42BOWIE';
ID TEXT TEXT_HASH
---------- --------- ----------------------------------------
44 42BOWIE A2C98939EDB479BC3EB0CDC560DDCD1575D47F62
431 429BOWIE A7E2B59E1429DB4964225E7A98A19998BC3D2AFD
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 143xd3cu22ja1, child number 0
-------------------------------------
select * from bowie where text between '429BOWIE' and '42BOWIE'
Plan hash value: 1845943507
---------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------
| 0 | SELECT STATEMENT | | | 208 (100)|
|* 1 | TABLE ACCESS FULL| BOWIE | 2 | 208 (2)|
---------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")<='42BOWIE' AND
INTERNAL_FUNCTION("TEXT")>='429BOWIE'))
7.如果做范围查询如何显示呢?期待作者的第2部分new extended columns in Part II.
自己也想一下,实际上作者的例子,text组成前面数字+BOWIE。前面5位具有很好的选择性。通过函数substr建立函数应该也可以,
自己测试看看。
SCOTT@test01p> create index i_bowie_text_substr_1_5 on bowie (substr(text,1,5));
Index created.
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 143xd3cu22ja1, child number 0
-------------------------------------
select * from bowie where text between '429BOWIE' and '42BOWIE'
Plan hash value: 1199225668
---------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 92 (100)|
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE | 2 | 92 (0)|
|* 2 | INDEX RANGE SCAN | I_BOWIE_TEXT_SUBSTR_1_5 | 450 | 3 (0)|
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")<='42BOWIE'
AND INTERNAL_FUNCTION("TEXT")>='429BOWIE'))
2 - access("BOWIE"."SYS_NC00004$">='429BO' AND "BOWIE"."SYS_NC00004$"<='42BOW')
--可以发现使用我建立的索引,看看使用大于的情况呢?
SCOTT@test01p> select * from bowie where text > 'zzz';
no rows selected
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 39wfprrkz66td, child number 0
-------------------------------------
select * from bowie where text > 'zzz'
Plan hash value: 1199225668
---------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 181 (100)|
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE | 1 | 181 (0)|
|* 2 | INDEX RANGE SCAN | I_BOWIE_TEXT_SUBSTR_1_5 | 900 | 4 (0)|
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")>'zzz'))
2 - access("BOWIE"."SYS_NC00004$">='zzz')
--//依旧可以使用我建立的函数索引,但是使用like情况如何呢?
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 1mq1xczjrz3uw, child number 0
-------------------------------------
select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%'
Plan hash value: 1845943507
---------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------
| 0 | SELECT STATEMENT | | | 208 (100)|
|* 1 | TABLE ACCESS FULL| BOWIE | 1 | 208 (2)|
---------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")
LIKE 'aaaaaaaaaaaaaaaaaaaaaa%'))
--//like 无效,不知道作者还有什么好方法,期待作者的第2部分,也许有更好的例子。
--//改写为范围查询也许是一个替换like的方法,但是不适合'%aaaa%'的情况。
SCOTT@test01p> select * from bowie where text between 'aaaaaaaaaaaaaaaaaaaaaa' and 'aaaaaaaaaaaaaaaaaaaaaa'||chr(255);
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID bkunhv8x64k0a, child number 1
-------------------------------------
select * from bowie where text between 'aaaaaaaaaaaaaaaaaaaaaa' and
'aaaaaaaaaaaaaaaaaaaaaa'||chr(255)
Plan hash value: 1199225668
---------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 2 (100)|
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE | 1 | 2 (0)|
|* 2 | INDEX RANGE SCAN | I_BOWIE_TEXT_SUBSTR_1_5 | 2 | 1 (0)|
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter((INTERNAL_FUNCTION("TEXT") AND
INTERNAL_FUNCTION("TEXT")>='aaaaaaaaaaaaaaaaaaaaaa' AND
INTERNAL_FUNCTION("TEXT")<='aaaaaaaaaaaaaaaaaaaaaa'))
2 - access("BOWIE"."SYS_NC00004$"='aaaaa')
--//总结:
--//BTW:如果字符字段很长,使用substr函数取前面选择性很强的几位,建立函数索引,有时候不失为一个方法,这样可以减少索引大小,
--//这种方法在10G,11G使用同样有效。
--//12C extended columns中提供的standard_hash函数,作为等值查询,不失为一个很好的选择。
[20180408]那些函数索引适合字段的查询.txt的更多相关文章
- 使用Oracle的instr函数与索引配合提高模糊查询的效率
使用Oracle的instr函数与索引配合提高模糊查询的效率 一般来说,在Oracle数据库中,我们对tb表的name字段进行模糊查询会采用下面两种方式:1.select * from tb wher ...
- MYSQ创建联合索引,字段的先后顺序,对查询的影响分析
MYSQ创建联合索引,字段的先后顺序,对查询的影响分析 前言 最左匹配原则 为什么会有最左前缀呢? 联合索引的存储结构 联合索引字段的先后顺序 b+树可以存储的数据条数 总结 参考 MYSQ创建联合索 ...
- MySQL 查询 存储过程 视图 触发器 函数 索引 建表语句 数据库版本 当前登录用户 当前数据库名称
MySQL 查询 存储过程 视图 触发器 函数 索引 建表语句 数据库版本 当前登录用户 当前数据库名称 INFORMATION_SCHEMA.TABLES INFORMATION_SCHEMA. ...
- [慢查优化]建索引时注意字段选择性 & 范围查询注意组合索引的字段顺序
文章转自:http://www.cnblogs.com/zhengyun_ustc/p/slowquery2.html 写在前面的话: 之前曾说过"不要求每个人一定理解 联表查询(join/ ...
- 注意使用 BTREE 复合索引各字段的 ASC/DESC 以优化 order by 查询效率
tbl_direct_pos_201506 表有 190 万数据.DDL: CREATE TABLE `tbl_direct_pos_201506` ( `acq_ins_code` char(13) ...
- Mybatis-Plus中使用max、sum聚合函数、只查询指定字段、查询语句多个OR处理
聚合函数查询 可以使用以下方法 QueryWrapper queryWrapper = new QueryWrapper<>(); queryWrapper.select(" I ...
- oracle的位图索引和函数索引
1.位图索引 位图索引适用于性别.婚姻状态.行政区等只有几列固定值的类型列,身份证号等就不适合位图索引,位图索引适用于静态数据,频繁更新的字段不适用建立位图索引,因为更新会导致索引块区的变更,还会引起 ...
- Mysql 索引原理《一》索引原理与慢查询2
一 索引未命中 并不是说我们创建了索引就一定会加快查询速度,若想利用索引达到预想的提高查询速度的效果,我们在添加索引时,必须遵循以下问题 1 范围问题,或者说条件不明确,条件中出现这些符号或关键字:& ...
- MySQL索引原理与慢查询
=========索引原理与慢查询======= 阅读目录 - 一.介绍 - 二.索引的原理 - 三.索引的数据结构 - 四.聚集索引与辅助索引 - 五.MyS ...
随机推荐
- 11-部署Heapster插件
配置和安装 Heapster 到 heapster release 页面 下载最新版本的 heapster. $ wget https://github.com/kubernetes/heapster ...
- [视频]K8飞刀 Discuz! X 系列(1.5 2.5 3.x)产品升级漏洞GetShell教程
K8飞刀 Discuz! X 系列(1.5 2.5 3.x)产品升级漏洞GetShell教程 https://pan.baidu.com/s/1bnv2euz
- EF code first,set composite primary key 复合key问题
环境: EF core 2.0 Net core 2.0 错误: 因实体定义了多个key,打开数据库时程序报以下错误 An unhandled exception occurred while pro ...
- ES启动报错最大进程数太少
[--16T18::,][INFO ][o.e.b.BootstrapChecks ] [node-] bound or publishing to a non-loopback address, e ...
- Python机器学习笔记:深入理解Keras中序贯模型和函数模型
先从sklearn说起吧,如果学习了sklearn的话,那么学习Keras相对来说比较容易.为什么这样说呢? 我们首先比较一下sklearn的机器学习大致使用流程和Keras的大致使用流程: skl ...
- #5 Python变量与输入输出
前言 学习一门编程语言,最基本的无非不过学习其变量规则.条件语句.循环语句和函数,接下来的几节将开始记录这些基本的语法,本节主要记录变量规则! 一.Python输入输出 在说Python变量之前,先补 ...
- Spring-IOC实现【01-XML配置方式】
IOC概念 IoC控制反转(IoC,Inversion of Control), 是一个概念,是一种思想.控制反转就 是对对象控制权的转移,从程序代码本身反转到了外部容器.把对象的创建.初始化. 销毁 ...
- 如何查看oracle用户具有的权限和角色
.查看所有用户: select * from dba_users; select * from all_users; select * from user_users; .查看用户或角色系统权限(直接 ...
- 【转】CentOS 7部署ASP.NET Core应用程序
很早就看过关于net core部署在Linux上的文章,自己也曾亲自将项目部署在Linux上,今天看到这篇文章,为其格式之工整而转! 1.环境准备 网上看了一下,Linux云服务器还挺贵的,那就只好先 ...
- c#连接访问数据库(菜鸡篇)
C#如何访问数据库(小白篇) 刚入坑不久学习的路上还是遇到了不小的问题,昨天学习C#的时候需要连接数据库获取数据. 网上有很多这样的文章,说实话对于我这样的小白还真是有点难理解,经过一番周折总算是了解 ...