pgsql: Stabilize test case added by commit f61db909d.

Stabilize test case added by commit f61db909d.

Buildfarm members ayu and tern have sometimes shown a different
plan than expected for this query. I'd been unable to reproduce
that before today, but I finally realized what is happening.
If there is a concurrent open transaction (probably an autovacuum
run in the buildfarm, but this can also be arranged manually),
then the index entries for the rows removed by the DELETE a few
lines up are not killed promptly, causing a change in the planner's
estimate of the extremal value of ft2.c1, which moves the rowcount
estimate for "c1 > 1100" by enough to change the join plan from
nestloop to hash.

To fix, change the query condition to "c1 > 1000", causing the
hash plan to be preferred whether or not a concurrent open
transaction exists. Since this UPDATE is tailored to be a no-op,
nothing else changes.

Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=ayu&dt=2021-06-09%2022%3A45%3A48
Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=ayu&dt=2021-06-13%2022%3A38%3A18
Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tern&dt=2021-06-20%2004%3A55%3A36

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5843659d091bfb6f2c60e010ea1fd00e55ee6ada

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 19 +++++++++++--------
contrib/postgres_fdw/sql/postgres_fdw.sql | 4 ++--
2 files changed, 13 insertions(+), 10 deletions(-)