pimp_db: add plpgsql function to extract random repos sample

sql/pimp_db.sql

@@ -26,11 +26,74 @@ CREATE TABLE repos_history (
 
 CREATE VIEW repo_creations AS
     SELECT today.ts :: date as date,
-	   today.repos - yesterday.repos as repos,
-	   today.fork_repos - yesterday.fork_repos as fork_repos,
-	   today.orig_repos - yesterday.orig_repos as orig_repos
+           today.repos - yesterday.repos as repos,
+           today.fork_repos - yesterday.fork_repos as fork_repos,
+           today.orig_repos - yesterday.orig_repos as orig_repos
     FROM repos_history today
     JOIN repos_history yesterday ON
-	 (yesterday.ts = (SELECT max(ts)
-			  FROM repos_history
-			  WHERE ts < today.ts));
+         (yesterday.ts = (SELECT max(ts)
+                          FROM repos_history
+                          WHERE ts < today.ts));
+
+-- -- return a random sample of repos, containing %percent repositories
+-- create or replace function repos_random_sample_array(percent real)
+-- returns setof repos as $$
+-- declare
+--     samples integer;
+--     repo repos%rowtype;
+--     ids integer[];
+-- begin
+--     select floor(count(*) / 100 * percent) into samples from repos;
+--     ids := array(select id from repos order by id);
+--     for i in 1 .. samples loop
+--      select * into repo
+--          from repos
+--          where id = ids[round(random() * samples)];
+--      return next repo;
+--     end loop;
+--     return;
+-- end
+-- $$
+-- language plpgsql;
+
+-- return a random sample of repositories
+create or replace function repos_random_sample(percent real)
+returns setof repos as $$
+declare
+    sample_size integer;
+begin
+    select floor(count(*) / 100 * percent) into sample_size from repos;
+    return query
+        select * from repos
+        order by random()
+        limit sample_size;
+    return;
+end
+$$
+language plpgsql;
+
+-- -- return a random sample of repositories
+-- create or replace function random_sample_sequence(percent real)
+-- returns setof repos as $$
+-- declare
+--     sample_size integer;
+--     seq_size integer;
+--     min_id integer;
+--     max_id integer;
+-- begin
+--     select floor(count(*) / 100 * percent) into sample_size from repos;
+--     select min(id) into min_id from repos;
+--     select max(id) into max_id from repos;
+--     seq_size := sample_size * 3;  -- IDs are sparse, generate a larger sequence
+--                                   -- to have enough of them
+--     return query
+--         select * from repos
+--         where id in
+--             (select floor(random() * (max_id - min_id + 1))::integer
+--                     + min_id
+--              from generate_series(1, seq_size))
+--         order by random() limit sample_size;
+--     return;
+-- end
+-- $$
+-- language plpgsql;