Computing the digits of π in PostgreSQL

pi_pg.sql

-- ref https://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/spigot.pdf

with recursive pigen(q, r, t, i, y, n) as (
  select 1::decimal, 180::decimal, 60::decimal, 2, 0::decimal, 0
  union all select 10*q*i*(2*i-1), 10*u*(q*(5*i-2)+r-y*t), t*u, i+1, y, n+1
  from (select q, r, t, i, n, 3*(3*i+1)*(3*i+2) as u, div(q*(27*i-12)+5*r,(5*t)) as y from pigen) as tmp
  where n < 1000
)
select string_agg(y::text, '') pi
from pigen where n > 0;