Why does this:
create or replace function dummy() returns double precision as $$ SELECT random() $$ LANGUAGE SQL COST 777; explain select dummy();
Result (cost=0.00..0.01 rows=1 width=0)
and not a cost of 777?
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I believe the costs refer to different units of measurement. The
cost when used in functions is the estimated execution cost in units of cpu_operator_cost, whereas the
EXPLAIN plan costs are units of disk page fetches (seq_page_cost).
For function cost details, see http://www.postgresql.org/docs/current/static/sql-createfunction.html
EXPLAIN plan costs, see http://www.postgresql.org/docs/current/static/using-explain.html