Postgres 14

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CREATE TABLE relation_tuple_transaction (
    id BIGSERIAL NOT NULL UNIQUE,
    timestamp TIMESTAMP WITHOUT TIME ZONE NOT NULL UNIQUE,
    CONSTRAINT pk_rttx PRIMARY KEY (id)
);
-- CREATE INDEX ix_relation_tuple_transaction_by_timestamp on relation_tuple_transaction using brin(id, timestamp);
CREATE INDEX ix_relation_tuple_transaction_by_timestamp on relation_tuple_transaction(timestamp);
INSERT INTO relation_tuple_transaction(timestamp) SELECT * FROM generate_series
        ( now() - interval '3 days'
        , now()
        , '1 second'::interval) dd
        ;
259201 rows affected

EXPLAIN ANALYZE
SELECT MIN(id), MAX(id) from relation_tuple_transaction WHERE timestamp >= now() - interval '5 seconds';

QUERY PLAN

Result  (cost=1.19..1.20 rows=1 width=16) (actual time=97.000..97.002 rows=1 loops=1)

  InitPlan 1 (returns $0)

    ->  Limit  (cost=0.42..0.59 rows=1 width=8) (actual time=96.933..96.935 rows=1 loops=1)

          ->  Index Scan using pk_rttx on relation_tuple_transaction  (cost=0.42..14904.25 rows=86024 width=8) (actual time=96.931..96.932 rows=1 loops=1)

                Index Cond: (id IS NOT NULL)

                Filter: ("timestamp" >= (now() - '00:00:05'::interval))

                Rows Removed by Filter: 259198

  InitPlan 2 (returns $1)

    ->  Limit  (cost=0.42..0.59 rows=1 width=8) (actual time=0.055..0.055 rows=1 loops=1)

          ->  Index Scan Backward using pk_rttx on relation_tuple_transaction relation_tuple_transaction_1  (cost=0.42..14904.25 rows=86024 width=8) (actual time=0.054..0.054 rows=1 loops=1)

                Index Cond: (id IS NOT NULL)

                Filter: ("timestamp" >= (now() - '00:00:05'::interval))

Planning Time: 0.706 ms

Execution Time: 97.051 ms

vacuum analyze;
EXPLAIN ANALYZE
SELECT MIN(id), MAX(id) from relation_tuple_transaction WHERE timestamp >= now() - interval '5 seconds';

QUERY PLAN

Aggregate  (cost=8.49..8.50 rows=1 width=16) (actual time=0.030..0.031 rows=1 loops=1)

  ->  Index Scan using ix_relation_tuple_transaction_by_timestamp on relation_tuple_transaction  (cost=0.42..8.48 rows=3 width=8) (actual time=0.023..0.024 rows=3 loops=1)

        Index Cond: ("timestamp" >= (now() - '00:00:05'::interval))

Planning Time: 0.434 ms

Execution Time: 0.105 ms