I tried pg_prewarm as suggested by Jeff Janes and it works - thanks a lot Jeff. Now the query planning is fast on the first execution.
Here is the list of tables that needed to be pre warmed (or you could just pre warm all the 'pg_%' tables. :-) ).
select pg_prewarm('pg_statistic');
select pg_prewarm('pg_trigger_tgrelid_tgname_index');
select pg_prewarm('pg_trigger');
select pg_prewarm('pg_statistic_relid_att_inh_index');
select pg_prewarm('pg_index_indrelid_index');
select pg_prewarm('pg_index_indexrelid_index');
select pg_prewarm('pg_index');
select pg_prewarm('pg_constraint_conrelid_index');
select pg_prewarm('pg_constraint');
select pg_prewarm('pg_class_relname_nsp_index');
select pg_prewarm('pg_class_oid_index');
select pg_prewarm('pg_attribute_relid_attnum_index');
select pg_prewarm('pg_attribute');
select pg_prewarm('pg_attrdef_adrelid_adnum_index');
select pg_prewarm('pg_attrdef');
select pg_prewarm('pg_amproc_fam_proc_index');
select pg_prewarm('pg_namespace_oid_index');
Regards,
