Tips N’ Tricks – count of all and just some

Let's assume you have very simple table with users:

# \d users
                           Table "public.users"
  Column   |  Type   |                     Modifiers
 id        | integer | not null default nextval('users_id_seq'::regclass)
 username  | text    | not null
 is_active | boolean | not null default true
    "users_pkey" PRIMARY KEY, btree (id)
    "users_username_key" UNIQUE, btree (username)

And you'd like to count all users, and know how many of them are active …

Continue reading Tips N' Tricks – count of all and just some

postgresql tips & tricks

cortilap @ freenode's #postgresql asked about how to create a check() that will allow only one of the columns to be not null.

it doesn't sound cool, let's see:

with 2 columns (a,b) you make a check: check ( (a is not null and b is null) or (a is null and b is not null) or (a is null and b is null))

whoa. and what about 3 columns? 4?

of course it creates some questions about the schema, but is there a way to do it? without such long checks?

one solution is to make a function to check it. but perhaps a simpler solution is possible?

luckily all of the fields are ints.

a quick think, and here we go:

check (coalesce(a*0, 1) + coalesce(b*0, 1) + coalesce(c*0, 1) > 1)

and what is the field was text? same thing, but instead of doing “X"*0, i would do “length(X)*0" 🙂

postgresql tips & tricks

mage_ from #postgresql had interesting problem today.

he has a table with 2 date fields, and he wants to have list of all years from both fields. together. as one list.

his approach:

select date_part('year', date1) from test
select date_part('year', date2) from test;

is hardly satisfactory – it takes too long.

any way to speed it up?
Continue reading postgresql tips & tricks