Question about the firstday sql and postgresql code, About the 6 hours.

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Description

Description of the issue, including:

• what you have tried
• references to similar issues
• queries demonstrating your question (if applicable)

I tried to get the firstday vitalsign from MIMIC-IV data. And I saw the official sql and postgresql code.
I found that both sql and postgresql code use a 6 Hours.

mimic-code/mimic-iv/concepts/firstday/first_day_vitalsign.sql

mimic-code/mimic-iv/concepts_postgres/firstday/first_day_vitalsign.sql

I see the code like follows:

-- THIS SCRIPT IS AUTOMATICALLY GENERATED. DO NOT EDIT IT DIRECTLY.
DROP TABLE IF EXISTS first_day_vitalsign; CREATE TABLE first_day_vitalsign AS
-- This query pivots vital signs and aggregates them
-- for the first 24 hours of a patient's stay.
SELECT
    ie.subject_id
    , ie.stay_id
    , MIN(heart_rate) AS heart_rate_min
    , MAX(heart_rate) AS heart_rate_max
    , AVG(heart_rate) AS heart_rate_mean
    , MIN(sbp) AS sbp_min
    , MAX(sbp) AS sbp_max
    , AVG(sbp) AS sbp_mean
    , MIN(dbp) AS dbp_min
    , MAX(dbp) AS dbp_max
    , AVG(dbp) AS dbp_mean
    , MIN(mbp) AS mbp_min
    , MAX(mbp) AS mbp_max
    , AVG(mbp) AS mbp_mean
    , MIN(resp_rate) AS resp_rate_min
    , MAX(resp_rate) AS resp_rate_max
    , AVG(resp_rate) AS resp_rate_mean
    , MIN(temperature) AS temperature_min
    , MAX(temperature) AS temperature_max
    , AVG(temperature) AS temperature_mean
    , MIN(spo2) AS spo2_min
    , MAX(spo2) AS spo2_max
    , AVG(spo2) AS spo2_mean
    , MIN(glucose) AS glucose_min
    , MAX(glucose) AS glucose_max
    , AVG(glucose) AS glucose_mean
FROM mimiciv_icu.icustays ie
LEFT JOIN mimiciv_derived.vitalsign ce
    ON ie.stay_id = ce.stay_id
        AND ce.charttime >= DATETIME_SUB(ie.intime, INTERVAL '6' HOUR)
        AND ce.charttime <= DATETIME_ADD(ie.intime, INTERVAL '1' DAY)
GROUP BY ie.subject_id, ie.stay_id;

I do not understand why there is a 6 Hour. Can we write it as follows?

-- THIS SCRIPT IS AUTOMATICALLY GENERATED. DO NOT EDIT IT DIRECTLY.
DROP TABLE IF EXISTS first_day_vitalsign; CREATE TABLE first_day_vitalsign AS
-- This query pivots vital signs and aggregates them
-- for the first 24 hours of a patient's stay.
SELECT
ie.subject_id
, ie.stay_id
, MIN(heart_rate) AS heart_rate_min
, MAX(heart_rate) AS heart_rate_max
, AVG(heart_rate) AS heart_rate_mean
, MIN(sbp) AS sbp_min
, MAX(sbp) AS sbp_max
, AVG(sbp) AS sbp_mean
, MIN(dbp) AS dbp_min
, MAX(dbp) AS dbp_max
, AVG(dbp) AS dbp_mean
, MIN(mbp) AS mbp_min
, MAX(mbp) AS mbp_max
, AVG(mbp) AS mbp_mean
, MIN(resp_rate) AS resp_rate_min
, MAX(resp_rate) AS resp_rate_max
, AVG(resp_rate) AS resp_rate_mean
, MIN(temperature) AS temperature_min
, MAX(temperature) AS temperature_max
, AVG(temperature) AS temperature_mean
, MIN(spo2) AS spo2_min
, MAX(spo2) AS spo2_max
, AVG(spo2) AS spo2_mean
, MIN(glucose) AS glucose_min
, MAX(glucose) AS glucose_max
, AVG(glucose) AS glucose_mean
FROM mimiciv_icu.icustays ie
LEFT JOIN mimiciv_derived.vitalsign ce
ON ie.stay_id = ce.stay_id
AND ce.charttime >= ie.intime
AND ce.charttime <= DATETIME_ADD(ie.intime, INTERVAL '1' DAY)
GROUP BY ie.subject_id, ie.stay_id;

Read mimic-code/mimic-iv/concepts/firstday/first_day_vitalsign.sql and mimic-code/mimic-iv/concepts_postgres/firstday/first_day_vitalsign.sql, then consult the linked MIMIC documentation. Determine why the six-hour lower bound is used and whether it should begin at ie.intime; done means providing a documented rationale or confirming a correction for both queries.