EXPLAIN: cost


oscon=>  explain select oid from pg_proc;
                        QUERY PLAN                         
-----------------------------------------------------------
 Seq Scan on pg_proc  (cost=0.00..72.92 rows=1492 width=4)

Two costs: startup cost and total cost
Floats, rounded to two places
Difference is the total run cost
Some operators have startup costs, some do not
Costs are only estimates, based on (sometimes) complex formulas
Costs are what drives the optimizer to pick one plan over another
Arbitrary number, related to page fetch (1.0) and CPU usage

Cost parameters

Parameter	Description	Default	vs. page read
page read	Cost to read fetch one page of data, by definition	1.00	-
cpu_tuple_cost	Cost of typical CPU time to process a tuple	0.01	100x quicker
cpu_index_tuple_cost	Cost of typical CPU time to process an index tuple	0.001	1000x quicker
cpu_operator_cost	Cost of CPU time to process a typical WHERE operator	0.0025	400x quicker
random_page_cost	Cost of a non-sequential page fetch	4	4x slower
effective_cache_size	Amount of cache = likelihood of finding a page in cache	1000	N/A

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