Arrays

Background

Arrays, as an extended type of database, are supported in PG, ClickHouse, Snowflake and other systems. They can be widely used in scenarios such as A/B tests, user tag analysis, and crowd profilings. StarRocks supports multidimensional array nesting, array slicing, comparison, and filtering.

Array usage

Array Definition

The following is an example of defining an array column in StarRocks:

-- One-dimensional arrays
create table t0(
  c0 INT,
  c1 `ARRAY<INT>`
)
duplicate key(c0);

-- Define nested arrays
create table t1(
  c0 INT,
  c1 `ARRAY<ARRAY<VARCHAR(10)>>`
)
duplicate key(c0);

As above, the array column is defined in the form of ARRAY and its TYPE has a default value of nullable. Currently, StarRocks does not support specifying TYPE as NOT NULL, but you can define the array itself as NOT NULL.

create table t2(
  c0 INT,
  c1 `ARRAY<INT>` NOT NULL
)
duplicate key(c0)

The array type has the following restrictions:

  • The array columns can be only defined in duplicate tables
  • Array columns cannot be used as key columns (may be supported later)
  • Array columns cannot be used as distribution columns
  • Array columns cannot be used as partition columns

Constructing arrays in SQL

Arrays can be constructed in SQL using brackets ("[" and "]"), with each array element separated by a comma (",")

select [1, 2, 3] as numbers;
select ["apple", "orange", "pear"] as fruit;
select [true, false] as booleans;

When array elements have different types, StarRocks will automatically derive the appropriate type (supertype)

select [1, 1.2] as floats;
select [12, "100"]; -- The results are ["12", "100"]

You can use pointed brackets (<>) to show the declared array type.

select `ARRAY<float>`[1, 2];
select `ARRAY<INT>`["12", "100"]; -- 结果是 [12, 100]

NULL can be included in the element

select [1, NULL];

For empty arrays, you can use pointed brackets to show the declared type, or you can write [] directly for StarRocks to infer the type based on the context. If StarRocks can’t infer the type, it will report an error.

select [];
select `ARRAY<VARCHAR(10)>`[];
select array_append([], 10);

Array import

There are three ways to write array values to StarRocks. Insert into is suitable for small-scale data testing. ORC Parquet inport and CSV import are suitable for large-scale data import.

  • INSERT INTO

    create table t0(c0 INT, c1 `ARRAY<INT>`)duplicate key(c0);
    INSERT INTO t0 VALUES(1, [1,2,3]);
  • Import from ORC Parquet file

    The array type in StarRocks corresponds to the list structure in ORC/Parquet format; no additional specification is needed. Currently ORC list structure can be imported directly.

  • Import from CSV file

    The array of CSV files is separated by comma by default. You can use stream load or routine load to import CSV files or CSV format data in Kafka.

Array element access

Access an element of an array using [ ] and subscripts, starting with 1.

mysql> select [1,2,3][1];

+------------+
| [1,2,3][1] |
+------------+
|          1 |
+------------+
1 row in set (0.00 sec)

If the subscript is 0 or a negative number, no error will be reported and NULL will be returned

mysql> select [1,2,3][0];

+------------+
| [1,2,3][0] |
+------------+
|       NULL |
+------------+
1 row in set (0.01 sec)

If the subscript exceeds the size of the array, NULL will be returned.

mysql> select [1,2,3][4];

+------------+
| [1,2,3][4] |
+------------+
|       NULL |
+------------+
1 row in set (0.01 sec)

For multidimensional arrays, the internal elements can be accessed recursively.

mysql(ARRAY)> select [[1,2],[3,4]][2];

+------------------+
| [[1,2],[3,4]][2] |
+------------------+
| [3,4]            |
+------------------+
1 row in set (0.00 sec)

mysql> select [[1,2],[3,4]][2][1];

+---------------------+
| [[1,2],[3,4]][2][1] |
+---------------------+
|                   3 |
+---------------------+
1 row in set (0.01 sec)