Example:
```
create table t1 (id integer primary key, ts real not null, a, b, c);
create table t2 (id integer primary key, ts real not null, d, e, f);
with nboe_id(id1, id2) -- find the Next Before or Equal keyset
as (
select t1.id as id1,
(
select t2.id
from t2
where t2.ts <= t1.ts
order by t2.ts desc
limit 1
)
from t1
),
naoe_id(id1, id2) -- find the Next After or Equal keyset
as (
select t1.id as id1,
(
select t2.id
from t2
where t2.ts >= t1.ts
order by t2.ts
limit 1
)
from t1
),
keyset_ts(id1, id2) -- combine the next before and next after keysets
as (
select id1,
id2
from nboe_id
where id2 is not null
union all
select id1,
id2
from naoe_id
where id2 is not null
order by id1
),
keyset(id1, id2, mintsd) -- generate the keyset with the minimum distance
as (
select id1,
id2,
min(abs(t1.ts - t2.ts))
from keyset_ts, t1, t2
where id1 == t1.id
and id2 == t2.id
group by id1
)
select t1.*,
t2.*
from keyset, t1, t2
where id1 == t1.id
and id2 == t2.id
;
```
NB: Not tested and I think I got the next before or equal and next after or equal order by's right.
NBB: Added slight optimizations