SET

Represents a collection of unordered, unique elements. Sets may contain only primitive types. In sets, unlike in arrays, element position is not meaningful.

Sets do not support LONG types (like LONG VARBINARY or LONG VARCHAR) or user-defined types (like Geometry).

If you populate a set from an array, Vertica sorts the values and removes duplicate elements. If you do not care about element position and plan to run queries that check for the presence of specific elements (find, contains), using a set could improve query performance.

Sets can be bounded, meaning they specify a maximum element count, or unbounded. Unbounded sets have a maximum binary size, which can be set explicitly or defaulted. See Limits on Element Count and Collection Size.

Restrictions

• Sets support only data of primitive types, for example, int, UUID, and so on.
• Bounds, if specified, are enforced for all operations that load or alter data. Unbounded sets may have as many elements as will fit in the allotted binary size.
• A set has a maximum binary size. If this size is not set when the set is defined, a default value is used.

Syntax for Direct Construction (Literals)

Use the SET keyword to construct a set value. Literal set values are contained in brackets. For example, to create a set of INT, you would do the following:

=> SELECT SET[1,2,3];
  set
-------
 [1,2,3]
(1 row)

You can explicitly convert an array to a set by casting, as in the following example:

=> SELECT ARRAY[1, 5, 2, 6, 3, 0, 6, 4]::SET[INT];
     set
-----------------
[0,1,2,3,4,5,6]
(1 row)

Notice that duplicate elements have been removed and the elements have been sorted.

Because the elements are known at the time you directly construct a set, these sets are implicitly bounded.

Syntax for Column Definition

Sets used in column definitions can be either bounded or unbounded. Bounded sets must specify a maximum number of elements. Unbounded sets can specify a maximum binary size for the set, or the value of DefaultArrayBinarySize is used. You can specify a bound or a binary size but not both. For more information about these values, see Limits on Element Count and Collection Size.

SET[data_type, max_elements]

SET[VARCHAR(50),100]

SET[data_type](max_size)

SET[VARCHAR(50)](32000)

SET[data_type]

SET[VARCHAR(50)]

SET[data_type](DefaultArrayBinarySize)

The following example defines a table with an unbounded set colum.

=> CREATE TABLE users
(
user_id INTEGER, 
display_name VARCHAR,
email_addrs SET[VARCHAR]
);

When you load array data into a column defined as a set, the array data is automatically converted to a set.

Set Output Format

Sets are shown in a JSON-like format, with comma-separated elements contained in brackets (like arrays). In the following example, the email_addrs column is a set.

=> SELECT custkey,email_addrs FROM customers LIMIT 4;
 custkey |                           email_addrs
---------+------------------------------------------------------------------------
 342176  | ["joe.smith@example.com"]
 342799  | ["bob@example,com","robert.jones@example.com"]
 342845  | ["br92@cs.example.edu"]
 342321  | ["789123@example-isp.com","sjohnson@eng.example.com","sara@johnson.example.name"]

Null Handling

Null semantics for collections are consistent with normal columns in most regards. See NULL Sort Order for more information on null-handling.

The null-safe equality operator (<=>) behaves differently from equality (=) when the collection is null rather than empty. Comparing a collection to NULL strictly is undefined.

=> SELECT ARRAY[1,3] = NULL;
?column?
----------

(1 row)

=> SELECT ARRAY[1,3] <=> NULL;
 ?column?
----------
 f
(1 row)

In the following example, the grants column in the table is null for employee 99.

=> SELECT grants = NULL FROM employees WHERE id=99;
 ?column?
----------

(1 row)

=> SELECT grants <=> NULL FROM employees WHERE id=99;
 ?column?
----------
 t
(1 row)

Empty arrays are not null and behave as expected.

=> SELECT ARRAY[]::ARRAY[INT] = ARRAY[]::ARRAY[INT];
 ?column?
----------
 t
(1 row)

Limits on Element Count and Collection Size

When declaring a collection type for a table column, you can limit either the number of elements or the total binary size of the collection. During query processing, Vertica always reserves the maximum memory needed for the column, based on either the element count or the binary size. If this size is much larger than your data actually requires, setting one of these limits can improve query performance by reducing the amount of memory that must be reserved for the column.

A bounded collection specifies a maximum element count. A value in a bounded collection column may contain fewer elements, but it may not contain more. Any attempt to insert more elements into a bounded collection than the declared maximum is an error. A bounded collection has a binary size that is the product of the data-type size and the maximum number of elements, possibly rounded up.

An unbounded collection specifies a binary size in bytes, explicitly or implicitly. It may contain as many elements as can fit in that binary size.

If a nested array or set specifies bounds for all dimensions, Vertica sets a single bound that is the product of the bounds. In the following example, the inner and outer arrays each have a bound of 10, but only a total element count of 100 is enforced.

ARRAY[ARRAY[INT,10],10]

If a nested collection specifies a bound for only the outer collection, it is treated as the total bound. The previous example is equivalent to the following:

ARRAY[ARRAY[INT],100]

You must either specify bounds for all nested collections or specify a bound only for the outer one. For any other distribution of bounds, Vertica treats the collection as unbounded.

You can change the bounds of a collection, including changing between bounded and unbounded collections, by casting.  See Casting.

Instead of specifying a bound, you can specify a maximum binary size for an unbounded collection. The binary size acts as an absolute limit, regardless of how many elements the collection contains. Collections that do not specify a maximum binary size use the value of DefaultArrayBinarySize. This size is set at the time the collection is defined and is not affected by later changes to the value of DefaultArrayBinarySize.

You can change the bounds or the binary size of an array column using ALTER TABLE as in the following example:

=> ALTER TABLE cust ALTER COLUMN orders SET DATA TYPE ARRAY[INTEGER](100);

If the change reduces the size of the collection and would result in data loss, the change fails.

You cannot set a maximum binary size for a bounded collection, only an unbounded one.

Casting

Casting a set casts each element of the set. You can therefore cast between data types following the same rules as for casts of scalar values.

You can cast both literal sets and set columns explicitly:

=> SELECT SET['1','2','3']::SET[INT];
   set
---------
[1,2,3]
(1 row)

=> CREATE TABLE transactions (tid INT, prod_ids SET[VARCHAR], quantities SET[VARCHAR(32)]);

=> INSERT INTO transactions VALUES (12345, SET['p1265', 'p4515'], SET['15','2']);
 
=> SELECT quantities :: SET[INT] FROM transactions;
 quantities
------------
   [15,2]
(1 row)

Assignment casts and implicit casts work the same way as for scalars.

You can perform explicit casts, but not implicit casts, between ARRAY and SET types. If the collection is unbounded and the data type does not change, the binary size is preserved. For example, if you cast an ARRAY[INT] to a SET[INT], the set has the same binary size as the array.

When casting an array to a set, Vertica first casts each element and then sorts the set and removes duplicates. If two source values are cast to the same target value, one of them will be removed. For example, if you cast an array of FLOAT to a set of INT, two values in the array might be rounded to the same integer and then be treated as duplicates. This also happens if the array contains more than one value that is cast to NULL.

If you cast from one element type to another, the resulting collection uses the default binary size. If this would cause the data not to fit, the cast fails.

Functions and Operators

See Collection Functions for a comprehensive list of functions that can be used to manipulate arrays and sets.

Collections support equality (=), inequality (<>), and comparison operators (<, <=, >, >=) between collections of the same type (arrays or sets). Comparisons follow these rules:

• A null collection is ordered last.
• Non-null collections are compared element by element, using the ordering rules of the element's data type. The relative order of the first pair of non-equal elements determines the order of the two collections.
• If all elements in both collections are equal up to the length of the shorter collection, the shorter collection is ordered before the longer one.
• If all elements in both collections are equal and the collections are of equal length, the collections are equal.

Collections can be used in the following ways:

• As the grouping column in a GROUP BY Clause.
• As the sort key in an ORDER BY Clause in a query, in an OVER clause (see Window Partitioning), or in a CREATE PROJECTION statement.
• As the sort key in the PARTITION BY part of an OVER clause.
• As a JOIN key (see Joined-Table).

Collections cannot be used as partition columns when creating tables. Collections cannot be used with analyze_statistics() or TopK projections.