Gbase8s composite index (I)

A simple index is an index created based on a column or function , The corresponding is the composite index . Composite index is also called composite index , That is, the index contains two or more columns .GBase8s The composite index of supports at most 16 Index columns , The total size of columns should be less than 390 bytes.

Examples of composite indexes are as follows ：

create index ix_items on items(stock_num,manu_code);

An example of a composite index is shown below ：

Using composite index can reduce the number of indexes , Enhance the uniqueness of the index , At the same time, it can improve the performance of join query based on multi column combination . for example , Indexes ix_items It can improve the performance of the following query statements ：

select * from items where stock_num=2 and manu_code='HSK';

meanwhile , You can also use this index for the following queries ：

select * from items where stock_num=2;

When creating a composite index , We need to consider the order of columns , It should be the order from the most commonly used column to the least commonly used column , The order of discrimination from high to low .

（1） It is necessary to consider the SQL Statement , Which predicates （ That is to say where Columns used by conditions ） Most of the times .

（2） You need to consider the differentiation of columns , The discrimination of a column is the number of all different values in the column . Generally, the columns with high discrimination are placed in front , Put the columns with low discrimination behind . Suppose there is A,B Two ,A The discrimination of is 100000, and B The discrimination of is 10000, Then don't consider other factors , It is recommended to use （A,B）.

When the above two conditions conflict , You need to consider the query that is emphasized in the application SQL And the actual test performance results , To determine the order of index columns . The above case, ,A The discrimination of is 100000,B The discrimination of is 10000, Suppose there is 50% Of SQL The statement only uses B Column query ,40% Used A,B Column , Then it is suggested to create （B,A） The order . here , If the application only queries A The performance requirements of columns are high , Then it needs to be considered in A Create a single index on .

For cases with little difference in discrimination , You need to determine the order of the index columns according to the actual test results . See the following GBase8s Composite index （ Two ）.