Why is PostgreSQL not using my indexes on a small table?

Question

I have the following table in PostgreSQL:

CREATE TABLE index_test
(
    id int PRIMARY KEY NOT NULL,
    text varchar(2048) NOT NULL,
    last_modified times         


        

Answer 1

The example you have found is for DB2, in pg you can use generate_series to do it. For example like this:

INSERT INTO index_test(data,last_modified,value,item_type) 
SELECT
    md5(random()::text),now(),floor(random()*100),md5(random()::text) 
    FROM generate_series(1,1000);
SELECT max(value) from index_test;

http://sqlfiddle.com/#!12/52641/3

The second query in above fiddle should use index only scan.

Answer 2

Maybe, the reason is that I have too few rows in my table?

Yes. For a total of 20 rows in a table a seq scan is always going to be faster than an index scan. Chances are that those rows are located in a single database block anyway, so the seq scan would only need a single I/O operation.

If you use

explain (analyze true, verbose true, buffers true) select ....

you can see a bit more details about what is really going on.

Btw: you shouldn't use text as a column name, as that is also a datatype in Postgres (and thus a reserved word).