Copy cartodb lib/sql scripts from CDB_CartodbfyTable branch

scripts-available/CDB_RandomTids.sql

@@ -0,0 +1,69 @@
+
+-- {
+-- 
+-- Return random TIDs in a table.
+--
+-- You can use like this:
+--
+--   SELECT * FROM lots_of_points WHERE ctid = ANY (
+--      ARRAY[ (SELECT CDB_RandomTids('lots_of_points', 100000)) ]
+--   );
+--
+-- NOTE:
+-- It currently doesn't really do it random, but in a
+-- equally-distributed way among all tuples.
+--
+--
+-- }{
+CREATE OR REPLACE FUNCTION CDB_RandomTids(in_table regclass, in_nsamples integer)
+  RETURNS tid[]
+AS $$
+DECLARE
+  class_info RECORD;
+  tuples_per_page INTEGER;
+  needed_pages INTEGER;
+  skip_pages INTEGER;
+  tidlist TID[];
+  pnrec RECORD;
+BEGIN
+
+  -- (#) estimate pages and tuples-per-page
+  --     HINT: pg_class.relpages, pg_class.reltuples
+  SELECT relpages, reltuples 
+    FROM pg_class WHERE oid = in_table
+    INTO class_info;
+
+  RAISE DEBUG 'Table % has % pages and % tuples',
+    in_table::text, class_info.relpages, class_info.reltuples;
+
+  IF in_nsamples > class_info.reltuples THEN
+    RAISE WARNING 'Table has less tuples than requested';
+    -- should just perform a sequencial scan here...
+  END IF;
+
+  tuples_per_page := floor(class_info.reltuples/class_info.relpages);
+  needed_pages := ceil(in_nsamples::real/tuples_per_page);
+
+  RAISE DEBUG '% tuples per page, we need % pages for % tuples',
+    tuples_per_page, needed_pages, in_nsamples;
+
+  -- (#) select random pages
+  --     TODO: see how good this is first
+
+  skip_pages := floor( (class_info.relpages-needed_pages)/(needed_pages+1) );
+
+  RAISE DEBUG 'we are going to skip % pages at each iteration',
+    skip_pages;
+
+  SELECT array_agg(t) FROM (
+    SELECT '(' || pn || ',' || tn || ')' as t
+    FROM generate_series(1, tuples_per_page) x(tn),
+         generate_series(skip_pages+1, class_info.relpages, skip_pages) y(pn) ) f
+        INTO tidlist;
+
+  RETURN tidlist;
+
+END
+$$ LANGUAGE 'plpgsql' STABLE STRICT;
+-- }
+