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