Question

I’ve got a table in which there are some columns with big text data. The query for 10 rows (table has only 31 records) takes more than 20 seconds. If I remove fields with big size, the query is executed quickly. The query for 1 row (by id) always executed quickly.

How can I do the query for many rows work more faster?

The query looks like this

SELECT DISTINCT (a.id), a.field_1, a.field_2, a.field_3
    , a.field_4, a.field_5, a.filed_6, ...  
FROM table_a a, table_b b 
WHERE a.field_8 = 'o'  
ORDER BY a.field_2 DESC 
LIMIT 10;

Answer 1

@a_horse already hinted at the likely syntax error. Try:

SELECT DISTINCT ON (a.id)  a.id, a.field_1, a.field_2, a.field_3, ...
FROM   table_a a
-- JOIN table_b b ON ???
WHERE  a.field_8 = 'o'  
ORDER  BY a.id, a.field_2 DESC 
LIMIT  10;

Note the bold emphasis and read up on the DISTINCT clause in the manual.

Also, an index on field_8 might help.
A multicolumn index on (field_8, id, field_2) might help even more, if you can narrow it down to that (and if that is the sort order you want, which I doubt).

If you want the result sorted by a.field_2 DESC first:
In PostgreSQL 9.1, if id is the primary key:

SELECT a.id, a.field_1, a.field_2, a.field_3, ...
FROM   table_a a
-- JOIN table_b b ON ???
WHERE  a.field_8 = 'o'
GROUP  BY a.id   -- primary key takes care of all columns in table a
ORDER  BY a.field_2 DESC 
LIMIT  10;