Azure CosmosDB User Defined Function returns records equal to the total number of records in the table - azure-cosmosdb

Question

I am just no understanding how user defined functions in CosmosDB works.
Why is the my UDF in Cosmos DB returning results equal to the number of records in the table?
My table has 4 records currently.
Here are the records -
{
"users": [
{
"partitionKey": "user",
"userPhoneNumber": "14168000000",
"userDisplayName": "Test User 1"
},
{
"partitionKey": "user",
"userPhoneNumber": "18055678978",
"userDisplayName": "Test User 2"
},
{
"partitionKey": "user",
"userPhoneNumber": "17202228799",
"userDisplayName": "Test User 3"
},
{
"partitionKey": "user",
"userPhoneNumber": "17780265987",
"userDisplayName": "Test User 4"
}
]
}
Here is my UDF -
function findUserByPhoneNumber(users, contactNumbers){
var result;
for (let i = 0; i < contactNumbers.length; i++) {
if (contactNumbers[i] == "14168000000") {
result = contactNumbers[i];
}
return result;
}
Here is my SQL -
SELECT udf.findUserByPhoneNumber(c,["14168000000","17200000000"]) FROM c
And here is the result I am getting -
[
{
"$1": "14168000000"
},
{
"$1": "14168000000"
},
{
"$1": "14168000000"
},
{
"$1": "14168000000"
}
]
I am only expecting one record to be returned.

Answer 1

The SELECT statement in your query will never help you filter items in your query. If you want to filter you should use WHERE in your query. While you could use a UDF in a WHERE statement that returns e.g. a boolean you should typically avoid it as the database can not use it indexes to efficiently retrieve your data and will have to use your UDF on every document to calculate if it should be retrieved.
For your scenario the query could be:
SELECT *
FROM c
WHERE ARRAY_CONTAINS(['14168000000', '17200000000'], c.userPhoneNumber)
The reason your UDF always returns 14168000000 is because you always use the array ['14168000000', '17200000000'] as input in your UDF and check if any of those values equals 14168000000 and return that.