聚合 + 最后 & 第一个 -> 丢失订单

Posted 2023-03-31

【中文标题】聚合 + 最后 & 第一个 -> 丢失订单

【英文标题】：Aggregation + last & first -> losing order

【发布时间】：2018-06-11 15:02:24

【问题描述】：

我正在尝试在 15 分钟的步长间隔内选择数据。主要的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”（如预期的那样）。 所以然后 ORDER BY "quarter" 4 带有 "quarter" == "0" 的行，从中选择最后一个值和第一个值。 这导致我无法保证基于时间戳的排序。

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

下面是表格示例：

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，以通知排序确实丢失了。

任何建议如何保持每 15 分钟的排序？

【问题讨论】：

First and last value of window function in one row in PostgreSQL的可能重复

【参考方案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，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，请参阅文档中的4.2.7. Aggregate Expressions。

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.