Recently, I worked on crawling the SQL Server 2005 Data using BDC and found out some interesting gotcha with the Incremental Crawl. BDC Configuration is nicely explained in the below article except some missing steps for SpecificFinder method:
Follow the above article to configure the BDC Search Crawl and then follow these steps for incremental crawl.
1. In SpecificFinder method, specify the TypeDescriptor for datetime field which is used for tracking the row data change.
<TypeDescriptor TypeName="System.DateTime" Name="UpdateTimeStamp" />
2. In SpecificFinder method, return the timestamp value. Full crawl crawls the datetime value and incremental crawl uses the datetime value for comparing the rows which gets updated post last full crawl and crawls only those rows.