I'm a bit surprised of that behavior thought, since it means that if we delete a row from table A all tables (B,C,D) with FK pointing to this table (A) must be scanned. If there is no index on those tables it means we gone do all Sequantial scans. Than can cause significant performance problem!!!.
Correct.
Is there a reason why implicit index aren't created when FK are declared.
Because it's not a requirement...
I looked into the documentation and I haven't found a way to tell postgresql to automatically create an index when creating la FK. Does it means I need to manage it EXPLICITLY with create index statement ? Is there another way ?
No other way - you need to explicitly create them. It's not that hard either to write a query to search the system catalogs for unindexed FK's.
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