Couchbase array index not getting used in the query - couchbase

Question

I have the following document structure:
{
"customerId": "",
"schemeId": "scheme-a",
"type": "account",
"events": [
{
"dateTime": "2019-03-14T02:23:58.573Z",
"id": "72998bbf-94a6-4031-823b-6c304707ad49",
"type": "DebitDisabled",
"authorisedId": ""
},
{
"dateTime": "2018-05-04T12:40:15.439Z",
"transactionReference": "005171-15-1054-7571-60990-20180503165536",
"id": "005171-15-1054-7571-60990-20180503165536-1",
"type": "Credit",
"authorisedId": ",
"value": 34,
"funder": "funder-a"
},
{
"dateTime": "2019-03-06T04:14:54.564Z",
"transactionReference": "000000922331",
"eventDescription": {
"language": "en-gb",
"text": "
},
"id": "000000922331",
"type": "Credit",
"authorisedId": "",
"value": 16,
"funder": "funder-b"
},
{
"dateTime": "2019-03-10T04:24:17.903Z",
"transactionReference": "000001510154",
"eventDescription": {
"language": "en-gb",
"text": ""
},
"id": "000001510154",
"type": "Credit",
"authorisedId": "",
"value": 10,
"funder": "funder-c"
}
]
}
And the following indexes :
CREATE INDEX `scheme-a_customers_index`
ON `default`(`type`,`schemeId`,`customerId`)
WHERE ((`schemeId` = "scheme-a") and (`type` = "account"))
WITH { "num_replica":1 }
CREATE INDEX `scheme-a_credits_index`
ON `default`(
`type`,
`schemeId`,
`customerId`,
(distinct (array (`e`.`funder`) for `e` in `events` when ((`e`.`type`) = "Credit") end))
)
WHERE ((`type` = "scheme") and (`schemeId` = "scheme-a"))
WITH { "num_replica":1 }
I am trying to query all the customerIds and events for each where type="credit" and funder like "funder%"
below is my query :
SELECT
customerId,
(ARRAY v.`value` FOR v IN p.events WHEN v.type = "Credit" AND v.funder like "funder%" END) AS credits
FROM default AS p
WHERE p.type = "account" AND p.schemeId = "scheme-a"
AND (ANY e IN p.events SATISFIES e.funder = "funder-a" END)
I am expecting the query to use the index scheme-a_credits_index, instead it is using scheme-a_customers_index. Can't understand why ! isn't the query supposed to use scheme-a_credits_index ?

Answer 1

Your query doesn't have predicate on customerId. So query can only push two predicates to indexers and both indexes are qualify. scheme-a_customers_index is more efficient because of number of entries in the index due to non array index.
You should try the following.
CREATE INDEX `ix1` ON `default`
(DISTINCT ARRAY e.funder FOR e IN events WHEN e.type = "Credit" END, `customerId`)
WHERE ((`schemeId` = "scheme-a") and (`type` = "account")) ;
SELECT
customerId,
(ARRAY v.`value` FOR v IN p.events WHEN v.type = "Credit" AND v.funder like "funder%" END) AS credits
FROM default AS p
WHERE p.type = "account" AND p.schemeId = "scheme-a"
AND (ANY e IN p.events SATISFIES e.funder LIKE "funder%" AND e.type = "Credit" END);