Cost of a query in PostgreSQL

Question

When 'Explain' is used along with the select statement in PostgreSQL, it gives the query plan as shown below

Seq Scan on employee  (cost=0.00..12.13 rows=1 width=438)
  Filter: ((lastname)::text = 'John'::text)

I couldn't understand what does '0.00..12.13' mean? Can anyone explain?

Answer 1

First number is startup cost. Second is estimated total cost.

According to documentation, the costs are measured in arbitrary units determined by the planner's cost parameters. Traditional practice is to measure the costs in units of disk page fetches; that is, seq_page_cost is conventionally set to 1.0 and the other cost parameters are set relative to that.