Qdabra Database Accelerator (DBXL) helps you quickly connect InfoPath forms to your SQL database and improve SharePoint submit and query performance. DBXL was first released in 2004 and supports many mission critical applications today. Qdabra recently released DBXL v2.4; for more information, please visit the product page on Qdabra.com.
Click here to see a list of available documentation for DBXL v2.4.
Edit: You can also refer to DBXL v2.5 documentation.