The following scenario describes a Job that retrieves people details from a lookup database, based on a join on the age. The main flow source data is read from a MySQL database table called people_age that contains people details such as numeric id, alphanumeric first name and last name and numeric age. The people age is either 40 or 60. The number of records in this table is intentionally restricted.
For more technologies supported by Talend, see Talend components.
The reference or lookup information is also stored in a MySQL database table called large_data_volume. This lookup table contains a number of records including the city where people from the main flow have been to. For the sake of clarity, the number of records is restricted but, in a normal use, the usefulness of the feature described in the example below is more obvious for very large reference data volume.
To optimize performance, a database connection component is used in the beginning of the Job to open the connection to the lookup database table in order not to do that every time we want to load a row from the lookup table.
An Expression Filter is applied to this lookup source flow, in order to select only data from people whose age is equal to 60 or 40. This way only the relevant rows from the lookup database table are loaded for each row from the main flow.
Therefore this Job shows how, from a limited number of main flow rows, the lookup join can be optimized to load only results matching the expression key.
Generally speaking, as the lookup loading is performed for each main flow row, this option is mainly interesting when a limited number of rows is processed in the main flow while a large number of reference rows are to be looked up to.
The join is solved on the age field. Then, using the relevant loading option in the tMap component editor, the lookup database information is loaded for each main flow incoming row.
For this Job, the metadata has been prepared for the source and connection components. For more information on how to set up the DB connection schema metadata, see the relevant section in the Talend Studio User Guide.
This Job is formed with five components, four database components and a mapping component.