SELECT source_id, get_time FROM bucket_2
:-] PREWHERE source_id=333415 AND get_time<='2020-06-26 09:00:00'
:-] order by source_id desc, get_time desc limit 1;

SELECT
    source_id,
    get_time
FROM bucket_2
PREWHERE (source_id = 333415) AND (get_time <= '2020-06-26 09:00:00')
ORDER BY
    source_id DESC,
    get_time DESC
LIMIT 1

┌─source_id─┬────────────get_time─┐
│    333415 │ 2020-06-26 06:43:54 │
└───────────┴─────────────────────┘

1 rows in set. Elapsed: 0.498 sec. Processed 603.49 thousand rows, 4.83 MB (1.21 million rows/s., 9.69 MB/s.)

SELECT get_time
FROM bucket_2
PREWHERE (source_id, get_time) IN
(
    SELECT
        source_id,
        max(get_time)
    FROM bucket_2
    PREWHERE (source_id = 333415) AND (get_time <= '2020-06-26 09:00:00')
    GROUP BY source_id
    UNION ALL
    SELECT
        source_id,
        min(get_time)
    FROM bucket_2
    PREWHERE (source_id = 333415) AND (get_time > '2020-06-26 09:00:00')
    GROUP BY source_id
)

┌────────────get_time─┐
│ 2020-06-26 06:43:54 │
└─────────────────────┘

1 rows in set. Elapsed: 0.050 sec. Processed 38.54 thousand rows, 308.30 KB (765.75 thousand rows/s., 6.13 MB/s.)