Efficient SQL query to find gap in consecutive numeric data (MySQL) - mysql

Question

I have a table with column "time" (INT unsigned), every row represents one second and I need to find gaps in time (missing seconds).
I have tried with this query (to find the first time before a gap):
SELECT t1.time
FROM `table` AS t1
LEFT JOIN `table` AS t2 ON t2.time=(t1.time+1)
WHERE t2.time IS NULL
ORDER BY TIME ASC
LIMIT 1
And it works but it's too slow for big tables (near 100M rows)
Is there some faster solution?
EXPLAIN query:
SHOW CREATE:
CREATE TABLE `candles` (
`time` int(10) unsigned NOT NULL,
`open` float unsigned NOT NULL,
`high` float unsigned NOT NULL,
`low` float unsigned NOT NULL,
`close` float unsigned NOT NULL,
`vb` int(10) unsigned NOT NULL,
`vs` int(10) unsigned NOT NULL,
`trades` int(10) unsigned NOT NULL,
PRIMARY KEY (`time`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

Answer 1

If DB version is 8.0, then The Recursive Common Table Expression might be used such as
WITH RECURSIVE cte AS
(
SELECT 1 AS n
UNION ALL
SELECT n + 1 AS value
FROM cte
WHERE cte.n < (SELECT MAX(time) FROM tab )
)
SELECT n AS gaps
FROM cte
LEFT JOIN tab
ON n=time
WHERE cte.n > (SELECT MIN(time) FROM tab )
AND time IS NULL
Demo

Answer 2

In MySQL 5.7, this is a use case where user variables might be helpful:
select max(time)
from (
select t.time, #rn := #rn + 1 as rn
from (select time from mytable order by time) t
cross join (select #rn := 0) r
) t
group by time - rn
This addresses the question as a gaps-and-islands problem. The idea is to identify groups of records where time increments without gaps (the islands). For this, we assign an incrementing id to each row, ordered by time; whenever the difference between time and the auto-increment changes, you know there is a gap.

Answer 3

With mysql 8, you can use LEAD():
select time from (
select time, lead(time, 1) over (order by time) next_time
from `table`
) t
where time+1 != next_time
In earlier versions, I might do something like:
select prev_time as time from (
select #prev_time+0 as prev_time,if(#prev_time:=time,time,time) as time
from (select #prev_time:=null) initvars
cross join (select time from `table` order by time) t
) t
where time != prev_time+1
Either will not include the greatest time, where your original query would have.
I think the group by required to treat it as a strict gaps and islands problem would be too expensive with that many records.
fiddle