Question

我想在PostgreSQL中编写一个程序，为我提供一些选定表的随机记录。我怎样才能做到这一点？我在网上找不到任何好的教程...

我知道我可以从PostgreSQL以多种方式获取随机记录。但我有很大的表，其中大多数都会导致性能问题。我也想学习一些新东西。

Answer 1

SELECT * FROM my_table
ORDER BY random() LIMIT 10;

Answer 2

我完成了类似的事情：

CREATE OR REPLACE FUNCTION get_random_id(in_table_name TEXT, want_elements INT4) RETURNS INT4[] as $BODY$

declare

    use_sql        TEXT;

    id_range       record;

    reply          INT4[];

    this_part      INT4[];

    try            INT4    := 0;

    i              INT4;

BEGIN

    use_sql := 'SELECT min(id), max(id) - min(id) + 1 as range FROM ' || quote_ident(in_table_name);

    EXECUTE use_sql INTO id_range;

    IF id_range.range < want_elements THEN

        raise exception 'not enough elements in % table.', in_table_name;

    END IF;

    WHILE ( try < 10 ) LOOP

        try := try + 1;

        this_part := '{}'::INT4[];

        WHILE ( coalesce(array_upper( this_part, 1 ), 0) < want_elements - coalesce(array_upper(reply, 1), 0)) LOOP

            i := floor( random() * id_range.range) + id_range.min;

            IF NOT i = ANY(this_part) THEN

                this_part := this_part || i;

            END IF;

        END LOOP;

        use_sql := 'SELECT id FROM ' || quote_ident(in_table_name) || ' WHERE id = ANY(' || quote_literal(this_part::TEXT) || '::INT4[])';

        for i IN EXECUTE use_sql LOOP

            reply := reply || i;

        END LOOP;

        IF (coalesce(array_upper(reply, 1), 0) = want_elements) THEN

            RETURN reply;

        END IF;

    END LOOP;

    RAISE EXCEPTION 'something strange happened - requested records not found in % tries', try;

END;

$BODY$ language plpgsql STABLE;

但是这个功能有一个问题。当我像这样使用它时：

select first_name from people WHERE (difference(first_name, 'first_name') < 2) and id = any(get_random_id('people', 3));

它有时会返回3条记录，有时会返回2或1 ...... :(

PostgreSQL程序将返回随机记录

2 个答案: