elasticsearch/docs/reference/query-languages/esql/esql-multivalued-fields.md

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Multivalued fields	https://www.elastic.co/guide/en/elasticsearch/reference/current/esql-multivalued-fields.html

{{esql}} multivalued fields [esql-multivalued-fields]

{{esql}} is fine reading from multivalued fields:

$$$esql-multivalued-fields-reorders$

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.

Duplicate values [esql-multivalued-fields-dups]

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

$$$esql-multivalued-fields-kwdups$

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 {{esql}} 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.

$$$esql-multivalued-fields-longdups$

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 {{esql}} 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:

$$$esql-multivalued-fields-longdups-tostring$

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 in a list [esql-multivalued-nulls]

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

$$$esql-multivalued-fields-multivalued-nulls$

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]],
  ]
}

Functions [esql-multivalued-fields-functions]

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

$$$esql-multivalued-fields-mv-into-null$

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:

• MV_AVG
• MV_CONCAT
• MV_COUNT
• MV_MAX
• MV_MEDIAN
• MV_MIN
• MV_SUM

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]
  ]
}