Re: timestamp arithmetics in C function - Mailing list pgsql-general

On 09/03/2018 09:11 AM, Lutz Gehlen wrote:
> Hello all,
> 
> unfortunately, I have so far not received a reply to my question
> below. I am well aware that no one has an obligation to reply; I was
> just wondering whether I phrased my question badly or whether there
> is anything else I could do to improve it.

Caveat, I am not a C programmer so I cannot comment on the correctness 
of the code. The question and it's phrasing look alright to me though. 
Your most recent post landed on a holiday(Labor Day) here in the States 
and therefore may have got lost in the return to work on Tuesday. 
Hopefully someone more knowledgeable then I will see this and comment on 
the C portion of your post.

> 
> Thanks for your help and best wishes,
> Lutz
> 
> 
> On Friday, 10.08.2018 09:05:40 Lutz Gehlen wrote:
>> Hello all,
>>
>> I am trying to implement a C function that accepts a date ("date"
>> in the sense of a type of information, not a postgres datatype)
>> as parameter (among others) and returns a certain point in time.
>> (The background is to calculate the time of dawn and dusk at the
>> given date.) Ideally, I would like to accept a timestamp value
>> and return another timestamp as result. I have implemented the
>> function, but I would like to ask advice on whether my
>> implementation is the recommended way to achieve this.
>>
>> To get started - since this is my first attempt at a C function in
>> postgres - I implemented a function that accepts the date as
>> three separate int32 values for year, month, and day and returns
>> the time of dawn as a float8 for the minutes since midnight (this
>> is what the implemented algorithm internally returns, anyway):
>>
>> ----
>> PG_FUNCTION_INFO_V1(dawn_utc);
>>
>> Datum dawn_utc(PG_FUNCTION_ARGS) {
>>    float8 lat              = PG_GETARG_FLOAT8(0);
>>    float8 lon              = PG_GETARG_FLOAT8(1);
>>    int32  year             = PG_GETARG_INT32(2);
>>    int32  month            = PG_GETARG_INT32(3);
>>    int32  day              = PG_GETARG_INT32(4);
>>    float8 solar_depression = PG_GETARG_FLOAT8(5);
>>
>>    // postgres-independent computation goes here
>>    float8 dawn_utc = calc_dawn_utc
>>      (lat, lon, year, month, day, solar_depression);
>>
>>    PG_RETURN_FLOAT8(dawn_utc);
>> }
>> ----
>>
>> This works fine. However, it would be more convenient if the
>> function would accept a date or timestamp value and return a
>> timestamp. So I modified the first part of the function like
>> this, based on code snippets I found in the postgres source code:
>>
>> ----
>> PG_FUNCTION_INFO_V1(dawn_utc);
>>
>> Datum dawn_utc(PG_FUNCTION_ARGS) {
>>    float8 lat              = PG_GETARG_FLOAT8(0);
>>    float8 lon              = PG_GETARG_FLOAT8(1);
>>    Timestamp timestamp     = PG_GETARG_TIMESTAMP(2);
>>    float8 solar_depression = PG_GETARG_FLOAT8(3);
>>
>>    struct pg_tm tt;
>>    struct pg_tm *tm = &tt;
>>    fsec_t       fsec;
>>
>>    if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)
>>      ereport(ERROR,
>>              (errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
>>               errmsg("timestamp out of range")));
>>
>>    // postgres-independent computation goes here
>>    float8 dawn_utc = calc_dawn_utc
>>      (lat, lon, tm->tm_year, tm->tm_mon, tm->tm_mday,
>> solar_depression;
>> ----
>>
>> For the second part of the function, I now have to add the
>> calculated number of minutes to the date portion of the timestamp
>> variable. One has to be aware that depending on the geographic
>> location dawn_utc can possibly be negative or larger than 1440
>> (i.e. 24h). I am not sure whether I should construct an interval
>> value from the number of minutes and add that to the timestamp. I
>> have not figured out how to do this, but decided to calculate a
>> new timestamp in a more fundamental way:
>>
>> ----
>>    tm->tm_sec  = 0;
>>    tm->tm_min  = 0;
>>    tm->tm_hour = 0;
>>    Timestamp result;
>>    if (tm2timestamp(tm, 0, NULL, &result) != 0)
>>      ereport(ERROR,
>>              (errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
>>               errmsg("timestamp out of range")));
>>
>> #ifdef HAVE_INT64_TIMESTAMP
>>    /* timestamp is microseconds since 2000 */
>>    result += dawn_utc * USECS_PER_MINUTE;
>> #else
>>    /* timestamp is seconds since 2000 */
>>    result += dawn_utc * (double) SECS_PER_MINUTE;
>> #endif
>>
>>    PG_RETURN_TIMESTAMP(result);
>> ----
>>
>> Again this code is based on what I found in the source code. It
>> seems to work correctly (at least on my development machine), but
>> I am wondering whether this is a safe and recommended way to
>> achieve this result or whether it is considered bad practice to
>> manipulate a timestamp on such fundamental level.
>>
>> Thank you for your advice and best wishes,
>> Lutz
> 
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com