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ES|QL multivalued fields

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ES|QL can read from multivalued fields:

				POST /mv/_bulk?refresh
					{ "index" : {} }
{ "a": 1, "b": [2, 1] }
{ "index" : {} }
{ "a": 2, "b": 3 }
				POST /_query
					{
  "query": "FROM mv | LIMIT 2"
}

Multivalued fields come back as a JSON array:

{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a", "type": "long"},
    { "name": "b", "type": "long"}
  ],
  "values": [
    [1, [1, 2]],
    [2,      3]
  ]
}

The relative order of values in a multivalued field is undefined. They’ll frequently be in ascending order but don’t rely on that.

Some field types, like keyword remove duplicate values on write:

				PUT /mv
					{
  "mappings": {
    "properties": {
      "b": {"type": "keyword"}
    }
  }
}
				POST /mv/_bulk?refresh
					{ "index" : {} }
{ "a": 1, "b": ["foo", "foo", "bar"] }
{ "index" : {} }
{ "a": 2, "b": ["bar", "bar"] }
				POST /_query
					{
  "query": "FROM mv | LIMIT 2"
}

And ES|QL sees that removal:

{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a", "type": "long"},
    { "name": "b", "type": "keyword"}
  ],
  "values": [
    [1, ["bar", "foo"]],
    [2,          "bar"]
  ]
}

But other types, like long don’t remove duplicates.

				PUT /mv
					{
  "mappings": {
    "properties": {
      "b": {"type": "long"}
    }
  }
}
				POST /mv/_bulk?refresh
					{ "index" : {} }
{ "a": 1, "b": [2, 2, 1] }
{ "index" : {} }
{ "a": 2, "b": [1, 1] }
				POST /_query
					{
  "query": "FROM mv | LIMIT 2"
}

And ES|QL also sees that:

{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a", "type": "long"},
    { "name": "b", "type": "long"}
  ],
  "values": [
    [1, [1, 2, 2]],
    [2,    [1, 1]]
  ]
}

This is all at the storage layer. If you store duplicate longs and then convert them to strings the duplicates will stay:

				PUT /mv
					{
  "mappings": {
    "properties": {
      "b": {"type": "long"}
    }
  }
}
				POST /mv/_bulk?refresh
					{ "index" : {} }
{ "a": 1, "b": [2, 2, 1] }
{ "index" : {} }
{ "a": 2, "b": [1, 1] }
				POST /_query
					{
  "query": "FROM mv | EVAL b=TO_STRING(b) | LIMIT 2"
}
		
{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a", "type": "long"},
    { "name": "b", "type": "keyword"}
  ],
  "values": [
    [1, ["1", "2", "2"]],
    [2,      ["1", "1"]]
  ]
}

null values in a list are not preserved at the storage layer:

				POST /mv/_doc?refresh
					{ "a": [2, null, 1] }
				POST /_query
					{
  "query": "FROM mv | LIMIT 1"
}
		
{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a", "type": "long"},
  ],
  "values": [
    [[1, 2]],
  ]
}

Unless otherwise documented functions will return null when applied to a multivalued field.

				POST /mv/_bulk?refresh
					{ "index" : {} }
{ "a": 1, "b": [2, 1] }
{ "index" : {} }
{ "a": 2, "b": 3 }
		
				POST /_query
					{
  "query": "FROM mv | EVAL b + 2, a + b | LIMIT 4"
}
		
{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a",   "type": "long"},
    { "name": "b",   "type": "long"},
    { "name": "b + 2", "type": "long"},
    { "name": "a + b", "type": "long"}
  ],
  "values": [
    [1, [1, 2], null, null],
    [2,      3,    5,    5]
  ]
}

Work around this limitation by converting the field to single value with one of:

				POST /_query
					{
  "query": "FROM mv | EVAL b=MV_MIN(b) | EVAL b + 2, a + b | LIMIT 4"
}
		
{
  "took": 28,
  "is_partial": false,
  "columns": [
    { "name": "a",   "type": "long"},
    { "name": "b",   "type": "long"},
    { "name": "b + 2", "type": "long"},
    { "name": "a + b", "type": "long"}
  ],
  "values": [
    [1, 1, 3, 2],
    [2, 3, 5, 5]
  ]
}