Subject: Re: very long record lines in expanded psql output
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Platon Pronko <platon7pronko@gmail.com>
Message-ID: <bb82e090-2587-5a64-003e-7203db5730fd@gmail.com>
Date: Thu, 5 Aug 2021 16:30:48 +0300
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Hi!

> a) with line cursor

Tried in different configurations, seems that line cursor works fine.

> b) format detection - I try to detect the header line - and I expect this
> line has the most length of all lines. I use a line with the same length as
> the column's type info (data, border).

I looked at pspg source, and here's what I found (please correct me if some/all
of my assumptions are wrong):

1. Input handling and format detection happens in table.c readfile().

2. There's some variables in DataDesc - maxx and maxbytes - that store the
longest line seen so far. But they are re-updated on each new row, so the fact
that header is shorter shouldn't affect them.

3. Expanded header detection is handled by is_expanded_header function. It has
ei_minx and ei_maxx return pointers, but when it is used from readfile() these
pointers are set to NULL in both cases - so header length is simply ignored.

> Did you test the wrapped format? It is working
> 
> \pset format wrapped
> \x
> select  * from pg_class;

There's no difference in outputs in wrapped format, so pspg behavior is also unaffected.

By the way, it seems that pspg recommends setting \pset border 2 anyway, so in vast
majority of cases there should be no possibility of difference at all - proposed patch
doesn't change output for \pset border 2 (because there's no sane way of making it
look okay in presence of long fields anyway).

Best regards,
Platon Pronko