What are Slowly Changing Dimensions?
Slowly Changing Dimensions (SCD) - dimensions that change slowly over time, rather than changing on regular schedule, time-base. In Data Warehouse there is a need to track changes in dimension attributes in order to report historical data. In other words, implementing one of the SCD types should enable users assigning proper dimension's attribute value for given date. Example of such dimensions are: customer, geography, employee.
- Type 0 - The passive method
- Type 1 - Overwrites the old value
- Type 2 - Creates a new additional record
- Type 3 - Adds a new column
- Type 4 - Uses a historical table
- Type 6 - Combines types 1,2,3 approaches (1+2+3=6)
There are many approaches how to deal with SCD. The most popular are:
accel-DS Shell Script Engine V2.0 is Now Availableaccel-DS Shell Script Engine can transform data using sql,files with most of the popular database and file formats used in Hadoop.
In Type 2 Slowly Changing Dimension, a new record is added to the table to represent the new information. Therefore, both the original and the new record will be present. The new record gets its own primary key.
In our example, recall we originally have the following table:
After Peter moved from Washington to Maine, we add the new information as a new row into the table:
Advantages:This allows us to accurately keep all historical information.
Disadvantages:This will cause the size of the table to grow fast. In cases where the number of rows for the table is very high to start with, storage and performance can become a concern.
This necessarily complicates the ETL process.
Usage:About 50% of the time.
When to use Type 2:Type 2 slowly changing dimension should be used when it is necessary for the data warehouse to track historical changes.
|1||Target_Table||Table where data should be loaded|
|2||Join_Columns||Enter source sql column and target table column to join those columns.|
|3||Mapped_Columns||Enter source sql column and target table column, source column data will be loaded in the respective target table column.|
|4||Source_SQL||Enter Source SQL query, which has new and updated records..|
|5||Target_Columns||Enter all the target table column names seperated by comma in the insersion order.|
|6||Eff_Date_Column||Enter target table column name, where record created date has to be loaded. Enter 'NA' if you don't want to track created date.|
|7||End_Date_Column||Enter target table column name where, recently record updated date has to be loaded. Enter 'NA' if you don't want to track updated date.|
|8||Latest_Rec_Column||Enter target table column name where flag specifies the record as latest or old. Enter 'NA' if you don't want to track latest record.|
|9||Data_Base_Name||Hadoop Database Name where the Target Table is located.|
|10||Transpose_Flag||Flag for Transpose. Enter 'Y' if you have Transpose column to load.|
|11||Transpose_Track_Column||Enter the source sql transpose column name, target table transpose column name and source column names to track transpose column (#This option is not applicable if (Arg) Transpose_Flag is 'N')|
|12||Audit_Columns||Audit columns delimited by ',' FORMAT : "creation_ts-CURRENT_TIMESTAMP,created_by-DEFAULT-VISHNU,process_control_id-UNIQUE_VALUE"|
|13||SQL_File_Location||Enter the path in which downloaded Type_2.sql file present.|
|14||Log_File||Enter the File path and name to log the script execution details.|
Target_Table="$Target_Table$" Source_SQL="$Source_SQL$" Mapped_Columns="$Mapped_Columns$" Join_Columns="$Join_Columns$" Target_Columns="$Target_Columns$" Data_Base_Name="$Data_Base_Name$" SQL_File_Location="$SQL_File_Location$" Log_File="$Log_File$" ./Type_1.sh
Target_Table="EDS_Request_Prod_KV" Join_Columns="ACCOUNT_ID,ACCOUNT_ID,SSN,SSN,SRC_SYSTEM,SRC_SYSTEM,ROW_TYPE,ROW_TYPE" Mapped_Columns="ACCOUNT_ID,ACCOUNT_ID,SSN,SSN,SRC_SYSTEM,SRC_SYSTEM,KV_COLUMN,KV_COLUMN,ROW_TYPE,ROW_TYPE,ADDRESS1,ADDRESS1,ADDRESS2,ADDRESS2,ADDRESS3,ADDRESS3,ADDRESS4,ADDRESS4,CITY,CITY,COUNTRY,COUNTRY,STATE,STATE,POSTAL,POSTAL" Source_SQL="SELECT * FROM cygnus.EDS_Request_STG_KV" Target_Columns="ACCOUNT_ID,SSN,SRC_SYSTEM,KV_COLUMN,ROW_TYPE,ADDRESS1,ADDRESS2,ADDRESS3,ADDRESS4,CITY,COUNTRY,STATE,POSTAL,as_of_date,creation_ts, created_by,process_control_id,effective_date,latestcol" Eff_Date_Column="effective_date" End_Date_Column="as_of_date" Latest_Rec_Column="latestcol" Data_Base_Name="cygnus" Transpose_Flag="Y" Transpose_Track_Column="kv_column,kv_column,ADDRESS1+ADDRESS2+ADDRESS3+ADDRESS4+CITY+COUNTRY+STATE+POSTAL,ADDRESS1+ADDRESS2+ADDRESS3+ADDRESS4+CITY+COUNTRY+STATE+POSTAL" Audit_Columns="creation_ts-CURRENT_TIMESTAMP,created_by-DEFAULT-VISHNU,process_control_id-UNIQUE_VALUE" SQL_File_Location="/home/cloudera/Desktop/Type_2" Log_File="/home/cloudera/Desktop/Type_2/type2_EDS_log.txt" ./Type_2.sh
Please ensure you read and understand the following general risk disclaimer:
IMPORTANT:THIS SOFTWARE END USER LICENSE AGREEMENT (“EULA”) IS A LEGAL AGREEMENT BETWEEN YOU AND PRODEN TECHNOLOGIES, INC. READ IT CAREFULLY BEFORE COMPLETING THE INSTALLATION PROCESS AND USING THE SOFTWARE. IT PROVIDES A LICENSE TO USE THE SOFTWARE AND CONTAINS WARRANTY INFORMATION AND LIABILITY DISCLAIMERS. BY INSTALLING AND USING THE SOFTWARE, YOU ARE CONFIRMING YOUR ACCEPTANCE OF THE SOFTWARE AND AGREEING TO BECOME BOUND BY THE TERMS OF THIS AGREEMENT. IF YOU DO NOT AGREE TO BE BOUND BY THESE TERMS, THEN SELECT THE "CANCEL" BUTTON. DO NOT PROCEED TO REGISTER & INSTALL THE SOFTWARE. LIABILITY DISCLAIMER•THE accel<>DS PROGRAM IS DISTRIBUTED "AS IS". NO WARRANTY OF ANY KIND IS EXPRESSED OR IMPLIED. YOU USE IT AT YOUR OWN RISK. NEITHER THE AUTHORS NOR PRODEN TECHNOLOGIES, INC. WILL BE LIABLE FOR DATA LOSS, DAMAGES AND LOSS OF PROFITS OR ANY OTHER KIND OF LOSS WHILE USING OR MISUSING THIS SOFTWARE.
RESTRICTIONS:You may not use, copy, emulate, clone, rent, lease, sell, modify, decompile, disassemble, otherwise reverse engineer, or transfer any version of the Software, or any subset of it, except as provided for in this agreement. Any such unauthorized use shall result in immediate and automatic termination of this license and may result in criminal and/or civil prosecution.
TERMS:This license is effective until terminated. You may terminate it by destroying the program, the documentation and copies thereof. This license will also terminate if you fail to comply with any terms or conditions of this agreement. You agree upon such termination to destroy all copies of the program and of the documentation, or return them to the author.
Download Data Ingestion Framework (Free)
SCD Type 2 Data Ingestion Scripts for Hadoop