PostgreSQL 儒略历学习资料

参考资料
Julian day number
pg source cdoe “src\backend\utils\adt\datetime.c”(317,1)

PostgreSQL Timestamp通过C函数转换为Date

Datum to_date1(PG_FUNCTION_ARGS) {
    Timestamp   timestamp = PG_GETARG_TIMESTAMP(0);
    DateADT     result;
    struct pg_tm tt,
        *tm = &tt;
    fsec_t      fsec;

    if (TIMESTAMP_IS_NOBEGIN(timestamp))
        DATE_NOBEGIN(result);
    else if (TIMESTAMP_IS_NOEND(timestamp))
        DATE_NOEND(result);
    else {
        if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)
            ereport(ERROR, (errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
                errmsg("timestamp out of range")));

        result = date2j(tm->tm_year, tm->tm_mon, tm->tm_mday) - POSTGRES_EPOCH_JDATE;
    }
    PG_RETURN_DATEADT(result);
}
Datum to_date2(PG_FUNCTION_ARGS) {
    TimestampTz timestamp = PG_GETARG_TIMESTAMPTZ(0);
    DateADT     result;
    struct pg_tm tt,
        *tm = &tt;
    fsec_t      fsec;
    int         tz;

    if (TIMESTAMP_IS_NOBEGIN(timestamp))
        DATE_NOBEGIN(result);
    else if (TIMESTAMP_IS_NOEND(timestamp))
        DATE_NOEND(result);
    else {
        if (timestamp2tm(timestamp, &tz, tm, &fsec, NULL, NULL) != 0)
            ereport(ERROR, (errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
                errmsg("timestamp out of range")));

        result = date2j(tm->tm_year, tm->tm_mon, tm->tm_mday) - POSTGRES_EPOCH_JDATE;
    }

    PG_RETURN_DATEADT(result);
}

timestamp2tm(timestamp dt, int *tzp, struct tm *tm, fsec_t *fsec, const char **tzn),函数实现文件为”\src\interfaces\ecpg\pgtypeslib\timestamp.c”(99,1):\
其中调用了函数
int j2date(int jd, int *year, int *month, int *day),实现文件为”src\backend\utils\adt\datetime.c”(317,1);
int date2j(int y, int m, int d),实现文件为”src\backend\utils\adt\datetime.c”(291,1);
其中一些常量不是太明白,经过搜索,请参看文档
Julian day number

select make_date(2000,1,1) - make_interval(days=>2451545),
make_date(1970,1,1) - make_interval(days=>2440588 )

output
4714-11-24 00:00:00 BC, 4714-11-24 00:00:00 BC

因此儒略历的日期偏移为

/* Julian-date equivalents of Day 0 in Unix and Postgres reckoning */
#define UNIX_EPOCH_JDATE        2440588 /* == date2j(1970, 1, 1) */
#define POSTGRES_EPOCH_JDATE    2451545 /* == date2j(2000, 1, 1) */

例如:2000年1月1日中午对应于JD = 2451545.0

drop function if exists d2j(date);
create or replace function d2j(date) 
    returns float8
as $$
    declare
        v_year integer;v_month integer;v_day integer;
        v_a integer; v_y integer;v_m integer;
        v_jdn integer;v_jd float8;
    begin
        v_year := date_part('year',$1);
        v_month := date_part('month',$1);
        v_day := date_part('day',$1);

        v_a := (14-v_month) / 12;
        v_y := v_year+4800-v_a;
        v_m := v_month+12*v_a-3;

        v_jdn := v_day + ((153 * v_m + 2)/5) + (365* v_y) + (v_y / 4) - (v_y / 100) +  (v_y / 400) - 32045;
        v_jd :=  v_jdn+ ( (0-12)/24 ) + (0/1440) + (0/86400);
        return  v_jd;
    end;
$$ language plpgsql;
--验证结果,可以通过"jd mon 7"来计算星期几，其中0表示星期一。
with cte as(
    select d2j(make_date(2000,1,1)) as f1
)select f1, (((f1::integer)%7) +1) as week from cte;