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PostgreSQL Composite Data Types

  • composite type represents the structure of a row or record; it is essentially just a list of field names and their data types. 
  • PostgreSQL allows composite types to be used in many of the same ways that simple types can be used. For example, a column of a table can be declared to be of a composite type.
Declaration of Composite Types:
Here are two simple examples of defining composite types:
CREATE TYPE complex AS (
    r       double precision,
    i       double precision

CREATE TYPE inventory_item AS (
    name            text,
    supplier_id     integer,
    price           numeric
The syntax is comparable to CREATE TABLE, except that only field names and types can be specified; no constraints (such as NOT NULL) can presently be included. Note that the AS keyword is essential; without it, the system will think a different kind of CREATE TYPE command is meant, and you will get odd syntax errors.
Having defined the types, we can use them to create tables:
CREATE TABLE on_hand (
    item      inventory_item,
    count     integer

INSERT INTO on_hand VALUES (ROW('fuzzy dice', 42, 1.99), 1000);
or functions:
CREATE FUNCTION price_extension(inventory_item, integer) RETURNS numeric
AS 'SELECT $1.price * $2' LANGUAGE SQL;

SELECT price_extension(item, 10) FROM on_hand;
Whenever you create a table, a composite type is also automatically created, with the same name as the table, to represent the table's row type. For example, had we said:
CREATE TABLE inventory_item (
    name            text,
    supplier_id     integer REFERENCES suppliers,
    price           numeric CHECK (price > 0)
then the same inventory_item composite type shown above would come into being as a byproduct, and could be used just as above. Note however an important restriction of the current implementation: since no constraints are associated with a composite type, the constraints shown in the table definition do not apply to values of the composite type outside the table. (A partial workaround is to use domain types as members of composite types.)

 Composite Value Input:

To write a composite value as a literal constant, enclose the field values within parentheses and separate them by commas. You can put double quotes around any field value, and must do so if it contains commas or parentheses. (More details appear below.) Thus, the general format of a composite constant is the following:
'( val1 , val2 , ... )'
An example is:
'("fuzzy dice",42,1.99)'
which would be a valid value of the inventory_item type defined above. To make a field be NULL, write no characters at all in its position in the list. For example, this constant specifies a NULL third field:
'("fuzzy dice",42,)'
If you want an empty string rather than NULL, write double quotes:
Here the first field is a non-NULL empty string, the third is NULL.
The ROW expression syntax can also be used to construct composite values. In most cases this is considerably simpler to use than the string-literal syntax since you don't have to worry about multiple layers of quoting. We already used this method above:
ROW('fuzzy dice', 42, 1.99)
ROW('', 42, NULL)
The ROW keyword is actually optional as long as you have more than one field in the expression, so these can simplify to:
('fuzzy dice', 42, 1.99)
('', 42, NULL)
 Accessing Composite Types:
To access a field of a composite column, one writes a dot and the field name, much like selecting a field from a table name. In fact, it's so much like selecting from a table name that you often have to use parentheses to keep from confusing the parser. For example, you might try to select some subfields from our on_hand example table with something like:
SELECT item.name FROM on_hand WHERE item.price > 9.99;
This will not work since the name item is taken to be a table name, not a column name of on_hand, per SQL syntax rules. You must write it like this:
SELECT (item).name FROM on_hand WHERE (item).price > 9.99;
or if you need to use the table name as well (for instance in a multitable query), like this:
SELECT (on_hand.item).name FROM on_hand WHERE (on_hand.item).price > 9.99;
Now the parenthesized object is correctly interpreted as a reference to the item column, and then the subfield can be selected from it.
Similar syntactic issues apply whenever you select a field from a composite value. For instance, to select just one field from the result of a function that returns a composite value, you'd need to write something like:
SELECT (my_func(...)).field FROM ...
Without the extra parentheses, this will generate a syntax error.

Modifying Composite Types:

Here are some examples of the proper syntax for inserting and updating composite columns. First, inserting or updating a whole column:
INSERT INTO mytab (complex_col) VALUES((1.1,2.2));

UPDATE mytab SET complex_col = ROW(1.1,2.2) WHERE ...;
The first example omits ROW, the second uses it; we could have done it either way.
We can update an individual subfield of a composite column:
UPDATE mytab SET complex_col.r = (complex_col).r + 1 WHERE ...;
Notice here that we don't need to (and indeed cannot) put parentheses around the column name appearing just after SET, but we do need parentheses when referencing the same column in the expression to the right of the equal sign.
And we can specify subfields as targets for INSERT, too:
INSERT INTO mytab (complex_col.r, complex_col.i) VALUES(1.1, 2.2);
Had we not supplied values for all the subfields of the column, the remaining subfields would have been filled with null values.

Composite Type Input and Output Syntax:

The external text representation of a composite value consists of items that are interpreted according to the I/O conversion rules for the individual field types, plus decoration that indicates the composite structure. The decoration consists of parentheses (( and )) around the whole value, plus commas (,) between adjacent items. Whitespace outside the parentheses is ignored, but within the parentheses it is considered part of the field value, and might or might not be significant depending on the input conversion rules for the field data type. For example, in:
'(  42)'
the whitespace will be ignored if the field type is integer, but not if it is text.
As shown previously, when writing a composite value you can write double quotes around any individual field value. You must do so if the field value would otherwise confuse the composite-value parser. In particular, fields containing parentheses, commas, double quotes, or backslashes must be double-quoted. To put a double quote or backslash in a quoted composite field value, precede it with a backslash. (Also, a pair of double quotes within a double-quoted field value is taken to represent a double quote character, analogously to the rules for single quotes in SQL literal strings.) Alternatively, you can avoid quoting and use backslash-escaping to protect all data characters that would otherwise be taken as composite syntax.
A completely empty field value (no characters at all between the commas or parentheses) represents a NULL. To write a value that is an empty string rather than NULL, write "".
The composite output routine will put double quotes around field values if they are empty strings or contain parentheses, commas, double quotes, backslashes, or white space. (Doing so for white space is not essential, but aids legibility.) Double quotes and backslashes embedded in field values will be doubled.


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