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 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 the use of mixed-type expressions even with user-defined types.
The PostgreSQL scanner/parser divides lexical elements into 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
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:
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.
PostgreSQL allows expressions with prefix and postfix unary (one-argument) operators, as well as binary (two-argument) operators. Like functions, operators can be overloaded, so the same problem of selecting the right operator exists.
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.
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
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
The system catalogs store information about which conversions, or casts, exist between which data types, 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 built-in types has been carefully crafted and is best not altered.)
An additional heuristic provided by the parser allows improved
determination of the proper casting behavior among groups of
types that have implicit casts. Data types are divided into
several basic type categories,
network, and user-defined. (For a list see
but note it is also possible to create custom type categories.)
Within each category there can be one or more preferred types, which are preferred when there
is a choice of possible types. With careful selection of
preferred types and available implicit casts, it is possible to
ensure that ambiguous expressions (those with multiple candidate
parsing solutions) can be resolved in a useful way.
All type conversion rules are designed with several principles in mind:
Implicit conversions should never have surprising or unpredictable outcomes.
There should be no extra overhead in the parser or executor if a query does not need implicit type conversion. That is, if a query is well-formed and the types already match, then the query should execute without spending extra time in the parser and without introducing unnecessary implicit conversion calls in 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 no longer do implicit conversion to use the old function.
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