CUBRID学习笔记 48查询优化

cubrid的中sql查询语法 查询优化

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------ 官方文档是英文的,看不明白可以参看ocracle的同类函数说明.很多都是一样的.

原文
http://www.cubrid.org/manual/93/en/sql/tuning.html

更新 UPDATE STATISTICS
UPDATE STATISTICS ON class-name[, class-name, ...] [WITH FULLSCAN];

UPDATE STATISTICS ON ALL CLASSES [WITH FULLSCAN];

UPDATE STATISTICS ON CATALOG CLASSES [WITH FULLSCAN];
CREATE TABLE foo (a INT, b INT);
CREATE INDEX idx1 ON foo (a);
CREATE INDEX idx2 ON foo (b);

UPDATE STATISTICS ON foo;
UPDATE STATISTICS ON foo WITH FULLSCAN;

UPDATE STATISTICS ON ALL CLASSES;
UPDATE STATISTICS ON ALL CLASSES WITH FULLSCAN;

UPDATE STATISTICS ON CATALOG CLASSES;
UPDATE STATISTICS ON CATALOG CLASSES WITH FULLSCAN;

Checking Statistics Information
csql下执行info stats table_name

查询计划
Viewing Query Plan

设置SET OPTIMIZATION LEVEL opt-level [;]
GET OPTIMIZATION LEVEL [ { TO | INTO } variable ] [;]

opt-level : A value that specifies the optimization level. It has the following meanings.

0: Does not perform query optimization. The query is executed using the simplest query plan. This value is used only for debugging.
1: Creates a query plan by performing query optimization and executes the query. This is a default value used in CUBRID, and does not have to be changed in most cases.
2: Creates a query plan by performing query optimization. However, the query itself is not executed. In general, this value is not used; it is used together with the following values to be set for viewing query plans.
257: Performs query optimization and outputs the created query plan. This value works for displaying the query plan by internally interpreting the value as 256+1 related with the value 1.
258: Performs query optimization and outputs the created query plan, but does not execute the query. That is, this value works for displaying the query plan by internally interpreting the value as 256+2 related with the value 2. This setting is useful to examine the query plan but not to intend to see the query results.
513: Performs query optimization and outputs the detailed query plan. This value works for displaying more detailed query plan than the value 257 by internally interpreting the value as 512+1.
514: Performs query optimization and outputs the detailed query plan. However, the query is not executed. This value works for displaying more detailed query plan than the value 258 by internally interpreting the value as 512+2.
Note If you configure the optimization level as not executing the query like 2, 258, or 514, all queries(not only SELECT, but also INSERT, UPDATE, DELETE, REPLACE, TRIGGER, SERIAL, etc.) are not executed.

SET OPTIMIZATION LEVEL 257;
-- csql> ;plan simple
SELECT /+ RECOMPILE / DISTINCT h.host_year, o.host_nation
FROM history h INNER JOIN olympic o
ON h.host_year = o.host_year AND o.host_year > 1950;

Query plan:

Sort(distinct)
Nested-loop join(h.host_year=o.host_year)
Index scan(olympic o, pk_olympic_host_year, (o.host_year> ?:0 ))
Sequential scan(history h)

Query Profiling

SET TRACE {ON | OFF} [OUTPUT {TEXT | JSON}]
ON: set on SQL trace.
OFF: set off SQL trace.
OUTPUT TEXT: print out as a general TEXT format. If you omit OUTPUT clause, TEXT format is specified.
OUTPUT JSON: print out as a JSON format.

csql> SET TRACE ON;
csql> SELECT /+ RECOMPILE / o.host_year, o.host_nation, o.host_city, SUM(p.gold)
FROM OLYMPIC o, PARTICIPANT p
WHERE o.host_year = p.host_year AND p.gold > 20
GROUP BY o.host_nation;
csql> SHOW TRACE;

Using SQL Hint

USE, FORCE, IGNORE INDEX

CREATE TABLE athlete2 (
code SMALLINT PRIMARY KEY,
name VARCHAR(40) NOT NULL,
gender CHAR(1),
nation_code CHAR(3),
event VARCHAR(30)
);
CREATE UNIQUE INDEX athlete2_idx1 ON athlete2 (code, nation_code);
CREATE INDEX athlete2_idx2 ON athlete2 (gender, nation_code);
Below two queries do the same behavior and they select index scan if the specified index, athlete2_idx2's scan cost is lower than sequential scan cost.

SELECT /+ RECOMPILE / *
FROM athlete2 USE INDEX (athlete2_idx2)
WHERE gender='M' AND nation_code='USA';

SELECT /+ RECOMPILE / *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX athlete2_idx2;
Below two queries do the same behavior and they always use athlete2_idx2

SELECT /+ RECOMPILE / *
FROM athlete2 FORCE INDEX (athlete2_idx2)
WHERE gender='M' AND nation_code='USA';

SELECT /+ RECOMPILE / *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX athlete2_idx2(+);
Below two queries do the same behavior and they always don't use athlete2_idx2

SELECT /+ RECOMPILE / *
FROM athlete2 IGNORE INDEX (athlete2_idx2)
WHERE gender='M' AND nation_code='USA';

SELECT /+ RECOMPILE / *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX athlete2_idx2(-);
Below query always do the sequential scan.

SELECT *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX NONE;

SELECT *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX athlete2.NONE;
Below query forces to be possible to use all indexes except athlete2_idx2 index.

SELECT *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX ALL EXCEPT athlete2_idx2;
When two or more indexes have been specified in the USING INDEX clause, the query optimizer selects the proper one of the specified indexes.

SELECT *
FROM athlete2 USE INDEX (athlete2_idx2, athlete2_idx1)
WHERE gender='M' AND nation_code='USA';

SELECT *
FROM athlete2
WHERE gender='M' AND nation_code='USA'
USING INDEX athlete2_idx2, athlete2_idx1;
When a query is run for several tables, you can specify a table to perform index scan by using a specific index and another table to perform sequential scan. The query has the following format.

SELECT *
FROM tab1, tab2
WHERE ...
USING INDEX tab1.idx1, tab2.NONE;
When executing a query with the index hint syntax, the query optimizer considers all available indexes on the table for which no index has been specified. For example, when the tab1 table includes idx1 and idx2 and the tab2 table includes idx3, idx4, and idx5, if indexes for only tab1 are specified but no indexes are specified for tab2, the query optimizer considers the indexes of tab2.

SELECT ...
FROM tab1, tab2 USE INDEX(tab1.idx1)
WHERE ... ;

SELECT ...
FROM tab1, tab2
WHERE ...
USING INDEX tab1.idx1;

Function-based Index

详细的优化请看原文.
其实主要是讲的 执行计划的含义.