We use to do MIN/MAX use IndexScan2 with LIMIT 1 in following cases.
– MIN argument needs to be constant or leading key only
– Predicate on leading key should not include NULL/MISSING
This is tricky to do in query engine use API2. One thing we can do is COUNT/MIN/MAX/COUNT(DISTINCT) attempt do API2 always. If more than 1 spans API2 will not be used.
Keshav Murthy any comments.

The rules we use to send limit one is some thing like below

GROUP BY is NIL (length of 0)
Number of aggregates are 1
KeyPos 0 (leading or first non equality)
ASC Index on Pos 0 MIN return first non-missing,non-null value is encountered scan can be stopped , if none return NULL
KeyPos 0 (leading or first non equality)
upto keyPos all leading keys all spans(across) must have have same lows and high , inclusive 3
Desc Index On Pos 0 MAX return first non-missing,non-null value is encountered scan can be stopped, if none return NULL
COUNT (DISTINCT <constant>)
After one item is scanned scan can be stopped

Hi Sitaram Vemulapalli, COUNT (DISTINCT <const_expr> ) will always return 1, pls confirm. 

Yes.

In that case, should COUNT (DISTINCT <const_expr> ) be pushed down, as it always results in 1 as response. 

It needs to pushdown, The response depends on what is predicate.
It all depends on group by, query predicate qualify any rows or not.

Okay Thanks. 

Build couchbase-server-5.5.0-2610 contains indexing commit dbd0c6daf8cef02aae8a10cc609c12a1a8dc7c37 with commit message:
MB-27861: Optimize MIN, MAX aggregate without group by
https://github.com/couchbase/indexing/commit/dbd0c6daf8cef02aae8a10cc609c12a1a8dc7c37

Build couchbase-server-6.0.0-1071 contains indexing commit dbd0c6daf8cef02aae8a10cc609c12a1a8dc7c37 with commit message:
MB-27861: Optimize MIN, MAX aggregate without group by
https://github.com/couchbase/indexing/commit/dbd0c6daf8cef02aae8a10cc609c12a1a8dc7c37

Had discussion about this with Deep and we have decided to make this optimization only for KeyPos = 0. For KeyPos > 0, we have logged a tracking bug MB-29605

Deepkaran Salooja does this need QE verification? if so, can you please provide some steps?

This was a small perf optimization. No functional verification needed. Dev has already verified the perf.

Rules were simplified due to MB-29553 with commit af8b9cd76f7561ac30a6272eef7e91ac240291f4

Sitaram Vemulapalli I need a clarification about below rules:

GROUP BY is NIL (length of 0)

Number of aggregates are 1

KeyPos 0 (leading or first non equality)

ASC Index on Pos 0 MIN return first non-missing,non-null value is encountered scan can be stopped , if none return NULL

KeyPos 0 (leading or first non equality)

upto keyPos all leading keys all spans(across) must have have same lows and high , inclusive 3

Desc Index On Pos 0 MAX return first non-missing,non-null value is encountered scan can be stopped, if none return NULL

COUNT (DISTINCT <constant>)

After one item is scanned scan can be stopped

if MIN Aggr is on KeyPos > 0 , for example, KeyPos = 3 and first three positions are: ASC, DESC< ASC, then do the rules still apply given that scan is equality low=high, incl 3 for first three positions (0, 1, 2) ?

if MIN Aggr is on KeyPos > 0 , for example, KeyPos = 3 and first three positions are: ASC, DESC< ASC, then do the rules still apply given that scan is equality low=high, incl 3 for first three positions (0, 1, 2) ?
keyPos=3 and first there is equal (first there asc,desc,asc doesn't matter because of equal) but 4th asc/desc matters. asc first, desc last value