When the Data Integration Service processes a Joiner transformation, it reads rows from both sources concurrently and builds the index and data cache based on the master rows.
The Data Integration Service then performs the join based on the detail source data and the cache data. The number of rows the Data Integration Service stores in the cache depends on the source data and whether you configure the Joiner transformation for sorted input.
To increase performance for an unsorted Joiner transformation, use the source with fewer rows as the master source. To increase performance for a sorted Joiner transformation, use the source with fewer duplicate key values as the master.