InSQL(Structured Query Language), the termcardinalityrefers to theuniquenessof data values contained in a particular column (attribute) of adatabasetable. The lower the cardinality, the more duplicated elements
For example: SELECT /*+ CARDINALITY ( [tablename] card ) */ 比方说如下表连接SQL, 往往会选择先读取 EMPLOYEES地记录,再通过USER_ID上的索引与表EMPLOYEES_ASSET进行连接。 SELECT * FROM EMPLOYEES WHERE level between :1 and :2 and USER_ID in (SELECT USER_ID FROM EMPLOYEES_ASSET WHERE IS_PROC...
对于列存储数据库,这些列可以大幅压缩以节约存储空间。 In SQL (Structured Query Language), the term cardinality refers to the uniqueness of data values contained in a particular column (attribute) of a database table. The lower the cardinality, the more duplicated elements in a column. Thus, a ...
Diving in the code Functional indexes Creating the function and the index Search using a function Another example and selectivity calculation Consequences of a bad estimate Last word Statistics, cardinality, selectivity SQL is a declarative language. It is a language where the user asks what he wa...
High CardinalityThe following is an example of High Cardinality, in which all values of a column must be unique.mysql> create table UniqueDemo1 -> ( -> id int, -> name varchar(100), ->UNIQUE(id,name) -> ); Query OK, 0 rows affected (0.81 sec ...
This example creates an index on thelast_namecolumn of theemployeestable, which can speed up queries filtering or sorting by last name. 2. Creating a Composite Index CREATEINDEXidx_full_nameONemployees(first_name,last_name); Here, a composite index is created on bothfirst_nameandlast_name, ...
Example Assume that type INT_ARRAY is defined as: CREATE TYPEINT_ARRAYAS INTEGER ARRAY[100] The SET statement in the following code fragment assigns variable LEN the value 4. BEGINDECLARELENINTEGER;DECLAREMYARRAY INT_ARRAY;SETMYARRAY =ARRAY[0,0,1,1];SETLEN =CARDINALITY(MYARRAY);END...
In these cases, cardinality is not particularly useful as there is o... 数据库 php sql sed oracle 原创 wx6323d657ef2b3 2022-09-16 13:03:56 66阅读 cbo_enable_low_cardinality_optimize Kubernetes是一种用于自动部署、扩展和管理容器化应用程序的开源平台。在Kubernetes中,我们经常需要对集群进行...
This example will ensure that fields with 100 or fewer distinct values will be extremely accurate. Although not guaranteed by the algorithm, if a cardinality is under the threshold, it is almost always 100% accurate. Cardinalities above this will begin to trade accuracy for memory savings, and ...
Let’s show it with an example. Let’s show it with the BUKRS column which contains the company code of a financial table of wide spread ISV application. Let’s assume we have one of those tables where we have 100 different company codes between 1000 and 1200 ...