Re: psql: Count all table footer lines in pager setup

v4-0001 LGTM.  It still gives me the same pager line count as my v2.

On 2025-10-02 23:00 +0200, Tom Lane wrote:
> Erik Wienhold <ewie@ewie.name> writes:
> > On 2025-10-02 00:25 +0200, Tom Lane wrote:
> >> I am not entirely sure that we should commit 0002 though; it may be
> >> that the savings is down in the noise anyway once you consider all
> >> the other work that happens while printing a big table.

I see larger gains for queries that produce cells with many newlines,
benchmarked with a variation of my test-psql-pager.py script from
upthread.  It measures the mtime of a file created before running psql
and a second file created by the pager using PAGER='touch somefile'
(that should cover the query execution time plus the formatting
overhead).  With that I consistently measure these times for psql's
normal output format:

Query: SELECT repeat(e'foo\n', :n_lines) FROM generate_series(1, :n_rows)

 n_lines |  n_rows  |  time[ms] (master)  | time[ms] (v4) | diff[%]
---------|----------|---------------------|---------------|---------
       1 |   100000 |              30.000 |        26.667 |  -11.11
       1 |  1000000 |             200.000 |       173.333 |  -13.33
       1 | 10000000 |            1889.998 |      1613.332 |  -14.63
      10 |    10000 |              16.667 |        13.333 |  -20.00
      10 |   100000 |              73.333 |        50.000 |  -31.82
      10 |  1000000 |             583.333 |       390.000 |  -33.14
     100 |     1000 |              16.667 |        10.000 |  -40.00
     100 |    10000 |              60.000 |        36.667 |  -38.89
     100 |   100000 |             490.000 |       280.000 |  -42.86
    1000 |      100 |              16.667 |        13.333 |  -20.00
    1000 |     1000 |              56.667 |        33.333 |  -41.18
    1000 |    10000 |             483.333 |       260.000 |  -46.21

Based on that I think you should push v4-0002.

> >> A positive reason not to take it is something I realized while
> >> checking test coverage: we never execute any of the
> >> maybe-use-the-pager branch of PageOutput in the regression tests,
> >> because isatty(stdout) will always fail.
> 
> I realized that we could address that if we really wanted to, using
> the same infrastructure as for tab-completion testing.  0001 and 0002
> attached are the same as before, and then I added 0003 which adds a
> draft-quality TAP test.  Code coverage checks show that this adds only
> about 10 more lines of coverage in psql proper, but in print.c most of
> the pager-related logic is now covered.

+1 on the TAP tests in general.  But I'm not too familiar with the TAP
infrastructure to provide any meaningful review, given that you consider
it just draft-quality.

> >> However, I'm not sure how useful it really is to have code coverage
> >> there, since by this very token the tests are paying zero attention
> >> to the value computed by count_table_lines.  It could be
> >> arbitrarily silly and we'd not see that.

Can we use something along the lines of my test-psql-pager.py to binary
search the line count at which psql uses the pager.  Because it would
only apply to systems that provide termios.h, we might be less
constrained to find a portable solution that tests whether the pager
triggers or not (which you noted in v4-0003.)

-- 
Erik Wienhold