Forcing a plan

• You can "strongly discourage" the planner from attempting certain plans
   
• SET enable_seqscan = false;
   
• True/false:
   
  • enable_seqscan
     
  • enable_indexscan
     
  • enable_sort
     
  • enable_nestloop
     
  • enable_hashjoin
     
  • enable_mergejoin
     
  • enable_tidscan
     
  • enable_hashagg (new in 7.4)

[costsize.c]

Cost		disable_cost = 100000000.0;

...

	if (!enable_sort)
		startup_cost += disable_cost;

oscon=> SET enable_seqscan = FALSE;
SET

oscon=>  EXPLAIN SELECT * FROM pg_class;
                            QUERY PLAN
--------------------------------------------------------------------
 Seq Scan on pg_class  (cost=100000000.00..100000004.24 R=124 W=151)

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