Foreign table performance issue / PostgreSQK vs. ORACLE

Hi!

I am struggling with the slow performance when running queries referring to foreign tables. – Yes, I know… - Please read the whole story!

The set-up basically is a production database and a reporting database. As names indicate, the production database is used for production, the reporting database is for analysis. On the reporting database, the only way to access product data is via foreign tables that link to the related production tables.

Now, while some queries on the reporting service run fine, some don't even return any data after hours.

However, the same set-up worked fine in Oracle before. Reporting wasn't always fast, but it delivered results in acceptable time. A query executed on the Oracle reporting server returns data in e.g. 30 seconds. But running the query translated to PostgreSQL on the PostgreSQL DB does not deliver a single row after hours (!) of run time.

So, I wonder: Is there a fundamental difference between Oracle database links and foreign tables in PostgreSQL that could explain the different run times? Could there be some tuning option in PostgreSQL to make queries via foreign tables faster (e.g. I heard about option fetch_size)?

Your pointes welcome!