What is the difference between external tables and web tables of Greenplum database?

Answer 1

The main difference between external tables and web tables are their data sources. External tables access static flat files, whereas web tables access dynamic data sources — either on a web server or by executing OS commands or scripts. When a query is planned using an external table, the external table is considered rescannable since the data is thought to be static for the course of the query. For web tables, the data is not rescannable because there is the possibility that the data could change during the course of the query’s execution.