postgresql What does afew mean?

Question

LINKs: https://www.2ndquadrant.com/wp-content/uploads/2019/05/sumtest.sql_.txt https://www.2ndquadrant.com/en/blog/join-lateral/

CREATE TABLE co_books (
    dd  numeric references books(dd),
    pname text references persons(pname),
    checked_out timestamptz,
    checked_in timestamptz);
Do $$
    DECLARE
    i int;
    afew int;
    p record;
    BEGIN
        FOR p IN SELECT pname FROM persons LOOP
            afew := (random() * 10)::integer;
            FOR i in 0..afew LOOP
                INSERT INTO co_books (dd, pname, checked_out) 
                VALUES 
                    ( (SELECT dd FROM books ORDER BY random() LIMIT 1), 
                         p.pname, /* hint: fix bad random data here */
                         (SELECT xtime FROM X2018 ORDER BY random() LIMIT 1)
                    );

            END LOOP;
        END LOOP;
    RETURN;
END;
$$;

If you follow the link, you can see it clearly that table person, column pname have 24 unique rows. After I executed the code block, then test how many rows in table: co_books. first time is 129 rows, then i delete the co_books, execute the code again, the rows is 156 rows, third time is 154 rows. So In this case, I think i don't understand the following part.

FOR p IN SELECT pname FROM persons LOOP
            afew := (random() * 10)::integer;
            FOR i in 0..afew LOOP

Laurenz Albe · Answer 1 · 2020-11-03T15:58:42.920

0

random() returns a random double precision number between 0 and 1.

(random() * 10)::integer

is a random integer between 0 and 10.

So the loop will perform somewhere between 1 and 11 INSERT statements on co_books.

So it is to be expected that the number of rows in the table varies every time the code is run.

edited Nov 03 '20 at 15:58

answered Nov 03 '20 at 13:25

Laurenz Albe

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So the overall rows shall be 24 * afew. – Mark Nov 03 '20 at 15:45
No, `afew` gets a new value each time. – Laurenz Albe Nov 03 '20 at 15:59
OK. Then It become a little bit of confusing. for exmpale if the code is like for i = 1 to 5 loop, for t = 1 to 4 loop, then the total inserted rows would be 4*5 = 20 rows. Here I don't know how many rows updated. – Mark Nov 05 '20 at 10:54
Right. But `random()` returns a different value every time it is called. – Laurenz Albe Nov 05 '20 at 11:05

postgresql What does afew mean?

1 Answers1