Index on timestamp field, and now() - Mailing list pgsql-general

Hello,

I have quite interesting problem. I have a table with a timestamp field.
I have an index on it. When I use constant date in where clause, PostgreSQL
uses index. But when I try to use now() there, it uses a sequence scan.
As far as I can understand in inside any query now() is a constant.
What is the problem here.

webmailstation=> explain analyze select id,send_date from queue where
send_date > now()+'20 year'::interval;
NOTICE:  QUERY PLAN:

Seq Scan on queue  (cost=0.00..10568.09 rows=30268 width=12) (actual
time=29347.82..29347.82 rows=0 loops=1)
Total runtime: 29347.97 msec

EXPLAIN

webmailstation=> explain analyze select id,send_date from queue where
send_date > '2022-1-1';
NOTICE:  QUERY PLAN:

Index Scan using queue_senddate_key on queue  (cost=0.00..37.71 rows=9
width=12) (actual time=29.20..29.20 rows=0 loops=1)
Total runtime: 30.26 msec

EXPLAIN

webmailstation=> set enable_seqscan to off;
SET VARIABLE
webmailstation=> explain analyze select id,send_date from queue where
send_date > now()+'20 year'::interval;
NOTICE:  QUERY PLAN:

Seq Scan on queue  (cost=100000000.00..100010568.09 rows=30268 width=12)
(actual time=24775.73..24775.73 rows=0 loops=1)
Total runtime: 24775.88 msec

EXPLAIN

webmailstation=>  select id,send_date from queue where send_date >
 '2022-1-1'; id | send_date
----+-----------
(0 rows)

webmailstation=> select count(*) from queue;
 count
-------
 89461
(1 row)

This is a statistics for this column.

webmailstation=# select * from pg_statistic where starelid=16729 and
staattnum=9;
 starelid | staattnum | stanullfrac | stawidth | stadistinct | stakind1 |
stakind2 | stakind3 | stakind4 | staop1 | staop2 | staop3 | staop4 |
stanumbers1 | stanumbers2 | stanumbers3 | stanumbers4 |

stavalues1

                   | stavalues2 | stavalues3 | stavalues4

----------+-----------+-------------+----------+-------------+----------+----
------+----------+----------+--------+--------+--------+--------+------------
-+-------------+-------------+-------------+---------------------------------
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-------------------------------------------------------+------------+--------
----+------------ 16729 |         9 |           0 |        8 |          -1 |
       2 | 3 |        0 |        0 |   1322 |   1322 |      0 |      0 |

| {-0.142043} |             |             | {"2001-12-27

08:01:04-05","2002-02-11 07:21:53.995488-05","2002-02-11
11:11:03.998751-05","2002-02-11 17:09:21.375388-05","2002-02-12
07:50:49.80789-05","2002-02-12 23:46:06-05","2002-02-13
23:58:54-05","2002-02-15 15:13:30.445873-05","2002-02-17
15:54:10.359982-05","2002-02-26 07:41:59-05","2003-07-21 13:10:06-04"} |

(1 row)

Any ides would be appreciated.

--
Denis