Couchbase DISTINCT very slow - couchbase

Question

I'm working through the free CB110 course on N1QL offered at learn.coucbase.com.
Following query in course's accompanying workbook takes 1 minute:
SELECT DISTINCT address.countryCode
FROM couchmusic2
WHERE email LIKE "%hotmail.com";
I have an gsi on email.
Following query takes milliseconds:
SELECT COUNT(*)
FROM couchmusic2
WHERE email LIKE "%hotmail.com";
which has me believe that DISTINCT is the problem.
EXPLAIN reveals this:
[
{
"plan": {
"#operator": "Sequence",
"~children": [
{
"#operator": "IndexScan",
"index": "idx_email",
"index_id": "c2e612a0d697d8b6",
"keyspace": "couchmusic2",
"namespace": "default",
"spans": [
{
"Range": {
"High": [
"[]"
],
"Inclusion": 1,
"Low": [
"\"\""
]
}
}
],
"using": "gsi"
},
{
"#operator": "Fetch",
"keyspace": "couchmusic2",
"namespace": "default"
},
{
"#operator": "Parallel",
"~child": {
"#operator": "Sequence",
"~children": [
{
"#operator": "Filter",
"condition": "((`couchmusic2`.`email`) like \"%hotmail.com\")"
},
{
"#operator": "InitialProject",
"distinct": true,
"result_terms": [
{
"expr": "((`couchmusic2`.`address`).`countryCode`)"
}
]
},
{
"#operator": "Distinct"
},
{
"#operator": "FinalProject"
}
]
}
},
{
"#operator": "Distinct"
}
]
},
"text": "\nSELECT DISTINCT address.countryCode \nFROM couchmusic2 \nWHERE email LIKE \"%hotmail.com\";"
}
]
Why is the query so slow? How do I speed this query up?

Answer 1

The count Query uses covered index.
Try the following index for DISTINCT Query.
CREATE INDEX ix1 ON couchmusic2(email,address.countryCode);
LIKE with leading % needs to complete indexScan. Check this out https://dzone.com/articles/a-couchbase-index-technique-for-like-predicates-wi

Answer 2

For pattern matching for all the strings ENDING with hotmail.com, do the following:
CREATE INDEX ix ON couchmusic2(SUBSTR(email, -11, 11), address.countryCode);
Modify the LIKE predicate to: WHERE SUBSTR(email, -11, 11) = "hotmail.com";
Obviously, this is suitable only for hotmail.com, you'll need another index.
Checkout TOKENS() function for more flexible way to index this.
To get the distinct values (when you have VERY large number of items compared to number of distinct values), try out the MIN() optimization along with it.
https://dzone.com/articles/count-amp-group-faster-using-n1ql