In many cases a user will not need
to understand the details of the type conversion mechanism.
However, the implicit conversions done by PostgreSQL
can affect the results of a query. When necessary, these results
can be tailored by using explicit type conversion.
SQL is a strongly typed language. That is, every data item
has an associated data type which determines its behavior and allowed usage.
PostgreSQL has an extensible type system that is
much more general and flexible than other SQL implementations.
Hence, most type conversion behavior in PostgreSQL
is governed by general rules rather than by ad hoc
heuristics. This allows
mixed-type expressions to be meaningful even with user-defined types.
The PostgreSQL scanner/parser divides lexical
elements into only five fundamental categories: integers, non-integer numbers,
strings, identifiers, and key words. Constants of most non-numeric types are
first classified as strings. The SQL language definition
allows specifying type names with strings, and this mechanism can be used in
PostgreSQL to start the parser down the correct
path. For example, the query
SELECT text 'Origin' AS "label", point '(0,0)' AS "value";
label | value
--------+-------
Origin | (0,0)
(1 row)
has two literal constants, of type text and point.
If a type is not specified for a string literal, then the placeholder type
unknown is assigned initially, to be resolved in later
stages as described below.
There are four fundamental SQL constructs requiring
distinct type conversion rules in the PostgreSQL
parser:
- Function calls
Much of the PostgreSQL type system is built around a
rich set of functions. Functions can have one or more arguments.
Since PostgreSQL permits function
overloading, the function name alone does not uniquely identify the function
to be called; the parser must select the right function based on the data
types of the supplied arguments.
- Operators
PostgreSQL allows expressions with
prefix and postfix unary (one-argument) operators,
as well as binary (two-argument) operators. Like functions, operators can
be overloaded, and so the same problem of selecting the right operator
exists.
- Value Storage
SQL INSERT and UPDATE statements place the results of
expressions into a table. The expressions in the statement must be matched up
with, and perhaps converted to, the types of the target columns.
- UNION, CASE, and related constructs
Since all query results from a unionized SELECT statement
must appear in a single set of columns, the types of the results of each
SELECT clause must be matched up and converted to a uniform set.
Similarly, the result expressions of a CASE construct must be
converted to a common type so that the CASE expression as a whole
has a known output type. The same holds for ARRAY constructs,
and for the GREATEST and LEAST functions.
The system catalogs store information about which conversions, called
casts, between data types are valid, and how to
perform those conversions. Additional casts can be added by the user
with the CREATE CAST command. (This is usually
done in conjunction with defining new data types. The set of casts
between the built-in types has been carefully crafted and is best not
altered.)
An additional heuristic is provided in the parser to allow better guesses
at proper behavior for SQL standard types. There are
several basic type categories defined: boolean,
numeric, string, bitstring, datetime, timespan, geometric, network,
and user-defined. Each category, with the exception of user-defined, has
one or more preferred types which are preferentially
selected when there is ambiguity.
In the user-defined category, each type is its own preferred type.
Ambiguous expressions (those with multiple candidate parsing solutions)
can therefore often be resolved when there are multiple possible built-in types, but
they will raise an error when there are multiple choices for user-defined
types.
All type conversion rules are designed with several principles in mind:
Implicit conversions should never have surprising or unpredictable outcomes.
User-defined types, of which the parser has no a priori knowledge, should be
"higher" in the type hierarchy. In mixed-type expressions, native types shall always
be converted to a user-defined type (of course, only if conversion is necessary).
User-defined types are not related. Currently, PostgreSQL
does not have information available to it on relationships between types, other than
hardcoded heuristics for built-in types and implicit relationships based on
available functions and casts.
There should be no extra overhead from the parser or executor
if a query does not need implicit type conversion.
That is, if a query is well formulated and the types already match up, then the query should proceed
without spending extra time in the parser and without introducing unnecessary implicit conversion
calls into the query.
Additionally, if a query usually requires an implicit conversion for a function, and
if then the user defines a new function with the correct argument types, the parser
should use this new function and will no longer do the implicit conversion using the old function.
