sql - 聚合 + last & first -> 丢失顺序

Question

我正在尝试在 15 分钟的步长间隔内选择数据。主要分组接缝按预期工作，但我在每 15 分钟组内失去秩序。原因是例如: 对于 time_stamp 在 0-14 分钟范围内的 4 个点 -> “floor(EXTRACT(minute FROM time_stamp)/15) AS quarter”，将返回值“0”(如预期)。因此，然后 ORDER BY "quarter"4 具有 "quarter"== "0"的行，从中选择最后一个值和第一个值。这导致了我无法保证基于时间戳的排序的情况。

SELECT
    first(value) as first_value,
    last(value) as last_value,
    CAST(EXTRACT(year FROM time_stamp) AS INTEGER) AS year,
    CAST(EXTRACT(month FROM time_stamp) AS INTEGER) AS month,
    CAST(EXTRACT(day FROM time_stamp) AS INTEGER) AS day,
    CAST(EXTRACT(hour FROM time_stamp) AS INTEGER) AS hour,
    floor(EXTRACT(minute FROM time_stamp) / 15) AS quarter,
FROM
    my_table
GROUP BY
    year,
    month,
    day,
    hour,
    quarter,
ORDER BY
    year,
    month,
    day,
    hour,
    quarter

下表示例:

CREATE TABLE my_table (
    id integer NOT NULL,
    time_stamp timestamp without time zone NOT NULL,
    value double precision NOT NULL,
);


CREATE SEQUENCE my_table_id_seq
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    CACHE 1;


ALTER TABLE ONLY my_table ALTER COLUMN id SET DEFAULT nextval('my_table_id_seq'::regclass);


ALTER TABLE ONLY my_table
    ADD CONSTRAINT my_table_pkey PRIMARY KEY (id);


CREATE INDEX ix_my_table_time_stamp ON my_table USING btree (time_stamp);

我还从查询中删除了“first”和“last”函数，以通知确实缺少排序。

关于如何在每 15 分钟的步骤中保持排序有什么建议吗？

Answer 1

没有标准的聚合函数 first() 和 last()，您可能是指用户定义的聚合函数，例如:

create or replace function first_agg(anyelement, anyelement)
returns anyelement language sql immutable strict
as $$ select $1; $$;

create or replace function last_agg(anyelement, anyelement)
returns anyelement language sql immutable strict
as $$ select $2; $$;

create aggregate first(anyelement) (
    sfunc = first_agg,
    stype = anyelement
);

create aggregate last(anyelement) (
    sfunc = last_agg,
    stype = anyelement
);

在聚合中使用order by，参见4.2.7. Aggregate Expressions在文档中。

SELECT
    first(value order by time_stamp) as first_value,
    last(value order by time_stamp) as last_value,
    CAST(EXTRACT(year FROM time_stamp) AS INTEGER) AS year,
    CAST(EXTRACT(month FROM time_stamp) AS INTEGER) AS month,
    CAST(EXTRACT(day FROM time_stamp) AS INTEGER) AS day,
    CAST(EXTRACT(hour FROM time_stamp) AS INTEGER) AS hour,
    floor(EXTRACT(minute FROM time_stamp) / 15) AS quarter
FROM
    my_table
GROUP BY
    year,
    month,
    day,
    hour,
    quarter
ORDER BY
    year,
    month,
    day,
    hour,
    quarter

DbFiddle.