8.16. Composite Types
A 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.
8.16.1. 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
NULL) can presently be included. Note that the
is essential; without it, the system will think a different kind
CREATE TYPE command is meant, and you will get odd syntax
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);
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.)
8.16.2. Constructing Composite Values
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:
val2, ... )'
An example is:
which would be a valid value of the
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:
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.
(These constants are actually only a special case of the generic type constants discussed in Section 22.214.171.124. The constant is initially treated as a string and passed to the composite-type input conversion routine. An explicit type specification might be necessary to tell which type to convert the constant to.)
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
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 be simplified to:
('fuzzy dice', 42, 1.99) ('', 42, NULL)
ROW expression syntax is discussed in more detail in Section 4.2.13.
8.16.3. 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
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
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.
The special field name
* means “all fields”, as
further explained in Section 8.16.5.
8.16.4. 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 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.
8.16.5. Using Composite Types in Queries
There are various special syntax rules and behaviors associated with composite types in queries. These rules provide useful shortcuts, but can be confusing if you don't know the logic behind them.
In PostgreSQL, a reference to a table name (or alias)
in a query is effectively a reference to the composite value of the
table's current row. For example, if we had a table
inventory_item as shown
above, we could write:
SELECT c FROM inventory_item c;
This query produces a single composite-valued column, so we might get output like:
c ------------------------ ("fuzzy dice",42,1.99) (1 row)
Note however that simple names are matched to column names before table
names, so this example works only because there is no column
c in the query's tables.
The ordinary qualified-column-name
can be understood as applying field
selection to the composite value of the table's current row.
(For efficiency reasons, it's not actually implemented that way.)
When we write
SELECT c.* FROM inventory_item c;
then, according to the SQL standard, we should get the contents of the table expanded into separate columns:
name | supplier_id | price ------------+-------------+------- fuzzy dice | 42 | 1.99 (1 row)
as if the query were
SELECT c.name, c.supplier_id, c.price FROM inventory_item c;
PostgreSQL will apply this expansion behavior to
any composite-valued expression, although as shown above, you need to write parentheses
around the value that
.* is applied to whenever it's not a
simple table name. For example, if
myfunc() is a function
returning a composite type with columns
c, then these two queries have the
SELECT (myfunc(x)).* FROM some_table; SELECT (myfunc(x)).a, (myfunc(x)).b, (myfunc(x)).c FROM some_table;
PostgreSQL handles column expansion by
actually transforming the first form into the second. So, in this
myfunc() would get invoked three times per row
with either syntax. If it's an expensive function you may wish to
avoid that, which you can do with a query like:
SELECT (m).* FROM (SELECT myfunc(x) AS m FROM some_table OFFSET 0) ss;
OFFSET 0 clause keeps the optimizer
from “flattening” the sub-select to arrive at the form with
multiple calls of
.* syntax results in
column expansion of this kind when it appears at the top level of
VALUES clause, or
a row constructor.
In all other contexts (including when nested inside one of those
.* to a composite value does not
change the value, since it means “all columns” and so the
same composite value is produced again. For example,
somefunc() accepts a composite-valued argument,
these queries are the same:
SELECT somefunc(c.*) FROM inventory_item c; SELECT somefunc(c) FROM inventory_item c;
In both cases, the current row of
passed to the function as a single composite-valued argument.
.* does nothing in such cases, using it is good
style, since it makes clear that a composite value is intended. In
particular, the parser will consider
refer to a table name or alias, not to a column name, so that there is
no ambiguity; whereas without
.*, it is not clear
c means a table name or a column name, and in fact
the column-name interpretation will be preferred if there is a column
Another example demonstrating these concepts is that all these queries mean the same thing:
SELECT * FROM inventory_item c ORDER BY c; SELECT * FROM inventory_item c ORDER BY c.*; SELECT * FROM inventory_item c ORDER BY ROW(c.*);
All of these
ORDER BY clauses specify the row's composite
value, resulting in sorting the rows according to the rules described
in Section 9.23.6. However,
inventory_item contained a column
c, the first case would be different from the
others, as it would mean to sort by that column only. Given the column
names previously shown, these queries are also equivalent to those above:
SELECT * FROM inventory_item c ORDER BY ROW(c.name, c.supplier_id, c.price); SELECT * FROM inventory_item c ORDER BY (c.name, c.supplier_id, c.price);
(The last case uses a row constructor with the key word
Another special syntactical behavior associated with composite values is
that we can use functional notation for extracting a field
of a composite value. The simple way to explain this is that
are interchangeable. For example, these queries are equivalent:
SELECT c.name FROM inventory_item c WHERE c.price > 1000; SELECT name(c) FROM inventory_item c WHERE price(c) > 1000;
Moreover, if we have a function that accepts a single argument of a composite type, we can call it with either notation. These queries are all equivalent:
SELECT somefunc(c) FROM inventory_item c; SELECT somefunc(c.*) FROM inventory_item c; SELECT c.somefunc FROM inventory_item c;
This equivalence between functional notation and field notation
makes it possible to use functions on composite types to implement
An application using the last query above wouldn't need to be directly
somefunc isn't a real column of the table.
Because of this behavior, it's unwise to give a function that takes a
single composite-type argument the same name as any of the fields of
that composite type. If there is ambiguity, the field-name
interpretation will be preferred, so that such a function could not be
called without tricks. One way to force the function interpretation is
to schema-qualify the function name, that is, write
8.16.6. 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 (
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:
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.
Remember that what you write in an SQL command will first be interpreted
as a string literal, and then as a composite. This doubles the number of
backslashes you need (assuming escape string syntax is used).
For example, to insert a
containing a double quote and a backslash in a composite
value, you'd need to write:
INSERT ... VALUES ('("\"\\")');
The string-literal processor removes one level of backslashes, so that
what arrives at the composite-value parser looks like
("\"\\"). In turn, the string
fed to the
text data type's input routine
"\. (If we were working
with a data type whose input routine also treated backslashes specially,
bytea for example, we might need as many as eight backslashes
in the command to get one backslash into the stored composite field.)
Dollar quoting (see Section 126.96.36.199) can be
used to avoid the need to double backslashes.
ROW constructor syntax is usually easier to work with
than the composite-literal syntax when writing composite values in SQL
ROW, individual field values are written the same way
they would be written when not members of a composite.