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

问题

I have the following table in PostgreSQL:

CREATE TABLE index_test
(
    id int PRIMARY KEY NOT NULL,
    text varchar(2048) NOT NULL,
    last_modified timestamp NOT NULL,
    value int,
    item_type varchar(2046)
);
CREATE INDEX idx_index_type ON index_test ( item_type );
CREATE INDEX idx_index_value ON index_test ( value )

I make the following selects:

explain select * from index_test r where r.item_type='B';
explain select r.value from index_test r where r.value=56;

The explanation of execution plan looks like this:

Seq Scan on index_test r  (cost=0.00..1.04 rows=1 width=1576)
    Filter: ((item_type)::text = 'B'::text)'

As far as I understand, this is a full table scan. The question is: why my indexes are not used?

May be, the reason is that I have too few rows in my table? I have only 20 of them. Could you please provide me with a SQL statement to easily populate my table with random data to check the indexes issue?

I have found this article: http://it.toolbox.com/blogs/db2luw/how-to-easily-populate-a-table-with-random-data-7888, but it doesn't work for me. The efficiency of the statement does not matter, only the simplicity.

回答1:

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).

回答2:

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.

来源：https://stackoverflow.com/questions/15474812/why-is-postgresql-not-using-my-indexes-on-a-small-table

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