Re: Understanding EXPLAIN

Robert Lichtenberger <r(dot)lichtenberger(at)synedra(dot)com> wrote:

> I am trying to fully understand, how costs for queries are computed.
> Taking the following example:
>
> CREATE TABLE test (name varchar(250) primary key) ;
> INSERT INTO test (name) VALUES(generate_series(1, 1000)::text) ;
> ANALYZE test ;
> EXPLAIN SELECT * FROM test WHERE name = '4' ;
>
> I am getting the output:
> Index Scan using test_pkey on test (cost=0.00..8.27 rows=1 width=3)
> Index Cond: ((name)::text = '4'::text)
>
> The server has default cost parameters
>
> The value I want to understand is 8.27. From reading the book
> "PostgreSQL 9.0 High Performance" I know, that we have one index page
> read (random page read, cost=4.0) and one database row read (random page
> read, cost=4.0) which comes up to a total of 8.0. But where are the
> missing 0.27 from?
>
> If I modify the example to insert 10,000 rows, the cost stays the same.
> Only if I go for 100,000 rows will the computed cost increase to 8.29.
>
> Can anybody enlighten me, please ;-).

There are some other costs, in your case cpu_tuple_cost and
cpu_index_tuple_cost.

Andreas
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