pgsql: btree_gin: Fix timestamp-vs-timestamptz comparisons near DST bou

btree_gin: Fix timestamp-vs-timestamptz comparisons near DST boundary.

To perform cross-type index scans with a timestamp index and a
timestamptz comparison value, btree_gin must convert the timestamptz
to timestamp to establish the starting point for the indexscan. This
is hazardous because timestamp_cmp_timestamptz is actually defined to
upconvert the timestamp to timestamptz and then compare timestamptz's.
When dealing with ambiguous timestamps near a fall-back DST transition,
the value may not round-trip, leading to selecting a scan start point
that misses some index entries that should match.

To fix that, move the converted value back an hour at a time until
it's clearly <= the given comparison value. (We could try to be
smarter, but this way seems more certain to work across all possible
DST behaviors.)

Another hazard is that for timestamptz's that are finite but close to
the upper range limit, the conversion may produce timestamp +Infinity,
again comparing greater than the given value. It's less clear whether
this can lead to missing matching values in practice, but it would at
least risk making the aforesaid correction loop very slow. So replace
+Infinity by the maximum finite timestamp.

The reverse case of timestamptz index and timestamp comparison value
is not at risk because upconverting the timestamp stays consistent
with what timestamptz_cmp_timestamp will do. Also, while
date-vs-timestamptz could theoretically be at risk, the conversion
error would have to be a full day which seems not to occur in reality.

Author: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAGRkXqQio=7PWgTfKvjyQY9-7A_0A36E3put1gxJeDypgqFSzw@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dbb9bac594914a82963220bf47ff2bdd16860d8b

Modified Files
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contrib/btree_gin/btree_gin.c | 28 ++++++++++++++++++++++++++++
contrib/btree_gin/expected/timestamp.out | 26 ++++++++++++++++++++++++++
contrib/btree_gin/sql/timestamp.sql | 9 +++++++++
3 files changed, 63 insertions(+)