Обсуждение: text and varchar are not equivalent

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/datatype-character.html
Description:

The documentation implies that the data types text and varchar are
equivalent, but this is not the case with this test in Postgresql version
16.
CREATE TEMPORARY TABLE test(ch char, vc varchar, txt text, txt0 text);
INSERT INTO test VALUES (' ', ' ', ' ','');
SELECT ch = vc AS ch_vc, ch = txt AS ch_txt, ch = txt0 AS ch_txt0,
vc = ch AS vc_ch, vc = txt AS vc_txt, vc = txt0 AS vc_txt0, 
txt = ch AS txt_ch, txt = vc AS txt_vc, txt = txt0 AS txt_txt0,
txt0 = ch AS txt0_ch, txt0 = vc AS txt0_vc, txt0 = txt AS txt0_txt
FROM test;

ch_vc    ch_txt    ch_txt0    vc_ch    vc_txt    vc_txt0    txt_ch    txt_vc    txt_txt0    txt0_ch    txt0_vc
txt0_txt
TRUE    FALSE    TRUE    TRUE    TRUE    FALSE    FALSE    TRUE    FALSE    TRUE    FALSE    FALSE

The tests are showing that the space character is treated differently in a
one character string. Whilst varchar = text, the comparison with char is
treated differently with text and varchar

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Fri, Feb 9, 2024, 10:12 PG Doc comments form <noreply@postgresql.org>
> wrote:
>> The documentation implies that the data types text and varchar are
>> equivalent, but this is not the case with this test in Postgresql version
>> 16.

> Fair point.  But I'd rather further emphasize that char should just be
> avoided so this and other unexpected outcomes simply do not manifest in a
> real database scenario.  Rather than try and document how odd it's behavior
> is when dealing with intra-textual type conversions.

Yeah, this is less about varchar acting oddly and more about char
acting oddly.  The short answer though is that text is a preferred
type, varchar is not, and that makes a difference when resolving
whether to apply text's or char's equality operator.  You can
detect how it's being handled with EXPLAIN:

regression=# explain verbose SELECT vc = ch AS vc_ch FROM test;
                          QUERY PLAN                           
---------------------------------------------------------------
 Seq Scan on pg_temp.test  (cost=0.00..17.88 rows=630 width=1)
   Output: ((vc)::bpchar = ch)
(2 rows)

regression=# explain verbose SELECT txt = ch AS txt_ch FROM test;
                          QUERY PLAN                           
---------------------------------------------------------------
 Seq Scan on pg_temp.test  (cost=0.00..19.45 rows=630 width=1)
   Output: (txt = (ch)::text)
(2 rows)

            regards, tom lane