postgresql - 「現在」からの絶対距離でタイムスタンプ（将来を含む）を並べ替える

Question

dateフィールドで私はこれを行うことができます：

ORDER BY ABS(expiry - CURRENT_DATE)

フィールドを使用timestampすると、次のエラーが発生します。

関数abs（interval）は存在しません

Answer 1

Use now() or CURRENT_TIMESTAMP for the purpose.

The reason for the different outcome of your queries is this:

When you subtract two values of type date, the result is an integer and abs() is applicable.
When you subtract two values of type timestamp (or just one is a timestamp), the result is an interval, and abs() is not applicable. You could substitute with a CASE expression:

ORDER BY CASE WHEN expiry > now() THEN expiry - now() ELSE now() - expiry END

Or you can extract() the unix epoch from the resulting interval like @Craig already demonstrated. I quote: "for interval values, the total number of seconds in the interval". Then you can use abs() again:

ORDER BY abs(extract(epoch from (expiry - now())));

age() would just add a more human readable representation to the interval by summing up days into months and years for for bigger intervals. But that's beside the point: the value is only used for sorting.

As your column is of type timestamp, you should use CURRENT_TIMESTAMP (or now()) instead of CURRENT_DATE, or you will get inaccurate results (or even incorrect for "today").

Answer 2

Compare with current_timestamp

SELECT the_timestamp > current_timestamp;

The age function is probably what you want when comparing them:

SELECT age(the_timestamp);

eg:

regress=# SELECT age(TIMESTAMP '2012-01-01 00:00:00');
      age       
----------------
 8 mons 17 days
(1 row)

If you want an absolute distance, use:

SELECT abs( extract(epoch from age(the_timestamp)) );

Answer 3

これは機能します（そして正しいソートを提供します）：

ABS(EXTRACT(DAY FROM expiry - CURRENT_TIMESTAMP))

残念ながら、Erwin Brandstetterが指摘したように、ソートの粒度が1日に低下します。