CREATE_CAST(7pg18)

📦 version 18.6, in opensuse-leap 16.0

💅 CREATE_CAST(7) 💅

okay so this is the manual page for CREATE_CAST, straight out of the PostgreSQL 18.6 Documentation. It's a long one bestie, so buckle up — there's more coming after this part 🫶

✨ NAME ✨

CREATE_CAST — define a new cast 💫

🎀 SYNOPSIS 🎀

CREATE CAST (source_type AS target_type)
    WITH FUNCTION function_name [ (argument_type [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (source_type AS target_type)
    WITHOUT FUNCTION
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (source_type AS target_type)
    WITH INOUT
    [ AS ASSIGNMENT | AS IMPLICIT ]

🦋 DESCRIPTION 🦋

CREATE CAST defines a new cast, and a cast is just... the vibe check between two data types? No wait, it's how you tell PostgreSQL to convert one data type into another. Iconic. For example:

SELECT CAST(42 AS float8);

converts the integer constant 42 to type float8 by invoking a previously specified function — in this case, float8(int4). (And if no suitable cast has been defined? The conversion fails. 💀 No cast, no conversion, periodt.)

Two types can be binary coercible, which means the conversion can happen "for free" without invoking any function at all. This requires that corresponding values use the same internal representation. For instance, the types text and varchar are binary coercible both ways. But bestie — binary coercibility is not necessarily a symmetric relationship. Example: the cast from xml to text can be performed for free in the present implementation, but the reverse direction requires a function that performs at least a syntax check. (Two types that are binary coercible both ways are also referred to as binary compatible.)

You can define a cast as an I/O conversion cast by using the WITH INOUT syntax. An I/O conversion cast is performed by invoking the output function of the source data type, and passing the resulting string to the input function of the target data type. In many common cases, this feature avoids the need to write a separate cast function for conversion. And here's the thing: an I/O conversion cast acts the same as a regular function-based cast — only the implementation is different. Same energy, different plumbing 💅

By default, a cast can be invoked only by an explicit cast request, that is an explicit CAST(x AS typename) or x::typename construct.

If the cast is marked AS ASSIGNMENT then it can be invoked implicitly when assigning a value to a column of the target data type. For example, supposing that foo.f1 is a column of type text, then:

INSERT INTO foo (f1) VALUES (42);

will be allowed if the cast from type integer to type text is marked AS ASSIGNMENT, otherwise not. (We generally use the term assignment cast to describe this kind of cast.)

If the cast is marked AS IMPLICIT then it can be invoked implicitly in any context, whether assignment or internally in an expression. (We generally use the term implicit cast to describe this kind of cast.) For example, consider this query:

SELECT 2 + 4.0;

The parser initially marks the constants as being of type integer and numeric respectively. There is no integer + numeric operator in the system catalogs, but there is a numeric + numeric operator. The query will therefore succeed if a cast from integer to numeric is available and is marked AS IMPLICIT — which in fact it is. The parser will apply the implicit cast and resolve the query as if it had been written

SELECT CAST ( 2 AS numeric ) + 4.0;

Now, the catalogs also provide a cast from numeric to integer. If that cast were marked AS IMPLICIT — which it is not 🙅 — then the parser would be faced with choosing between the above interpretation and the alternative of casting the numeric constant to integer and applying the integer + integer operator. Lacking any knowledge of which choice to prefer, it would give up and declare the query ambiguous. The fact that only one of the two casts is implicit is the way in which we teach the parser to prefer resolution of a mixed numeric-and-integer expression as numeric; there is no built-in knowledge about that. The parser is literally just out here trusting your cast labels, bestie 😭

It is wise to be conservative about marking casts as implicit. An overabundance of implicit casting paths can cause PostgreSQL to choose surprising interpretations of commands, or to be unable to resolve commands at all because there are multiple possible interpretations. A good rule of thumb is to make a cast implicitly invokable only for information-preserving transformations between types in the same general type category. For example, the cast from int2 to int4 can reasonably be implicit, but the cast from float8 to int4 should probably be assignment-only. Cross-type-category casts, such as text to int4, are best made explicit-only. 🚨
Note Sometimes it is necessary for usability or standards-compliance reasons to provide multiple implicit casts among a set of types, resulting in ambiguity that cannot be avoided as above. The parser has a fallback heuristic based on type categories and preferred types that can help to provide desired behavior in such cases. See CREATE TYPE (CREATE_TYPE(7)) for more information.

To be able to create a cast, you must own the source or the target data type and have USAGE privilege on the other type. To create a binary-coercible cast, you must be superuser. (This restriction is made because an erroneous binary-coercible cast conversion can easily crash the server. 😱 Not a drill.)

🌈 PARAMETERS 🌈

  • source_type — the name of the source data type of the cast. The one we're leaving from ✈️
  • target_type — the name of the target data type of the cast. The one we're arriving at 🛬
  • function_name[(argument_type [, ...])] — the function used to perform the cast. The function name can be schema-qualified. If it is not, the function will be looked up in the schema search path. The function's result data type must match the target type of the cast. Its arguments are discussed below. If no argument list is specified, the function name must be unique in its schema.
  • WITHOUT FUNCTION — indicates that the source type is binary-coercible to the target type, so no function is required to perform the cast. Free conversion, no middleman 💅
  • WITH INOUT — indicates that the cast is an I/O conversion cast, performed by invoking the output function of the source data type, and passing the resulting string to the input function of the target data type.
  • AS ASSIGNMENT — indicates that the cast can be invoked implicitly in assignment contexts.
  • AS IMPLICIT — indicates that the cast can be invoked implicitly in any context.

Cast implementation functions can have one to three arguments. Let's break it down, because this is where it gets spicy 🌶️

  • The first argument type must be identical to or binary-coercible from the cast's source type.
  • The second argument, if present, must be type integer; it receives the type modifier associated with the destination type, or -1 if there is none.
  • The third argument, if present, must be type boolean; it receives true if the cast is an explicit cast, false otherwise. (Bizarrely, the SQL standard demands different behaviors for explicit and implicit casts in some cases. This argument is supplied for functions that must implement such casts. It is not recommended that you design your own data types so that this matters. 🙈)

The return type of a cast function must be identical to or binary-coercible to the cast's target type.

Ordinarily a cast must have different source and target data types. However, it is allowed to declare a cast with identical source and target types if it has a cast implementation function with more than one argument. This is used to represent type-specific length coercion functions in the system catalogs. The named function is used to coerce a value of the type to the type modifier value given by its second argument.

When a cast has different source and target types and a function that takes more than one argument, it supports converting from one type to another and applying a length coercion in a single step. When no such entry is available, coercion to a type that uses a type modifier involves two cast steps — one to convert between data types and a second to apply the modifier. Two steps, bestie. The long way around 🚶♀️

A cast to or from a domain type currently has no effect. Casting to or from a domain uses the casts associated with its underlying type.

📝 NOTES 💅

Okay so first thing's first: when you wanna yeet a user-defined cast out of existence, that's DROP CAST's whole job. Not yours. Don't try to manifest it away.

And bestie, listen — if you want types to convert both ways, you gotta declare casts both ways. Explicitly. There's no auto-reciprocity situation happening here. The cast does not love you back unless you write it down. 😭

🎀 the string-type thing (iykyk) 🎀

It's normally not necessary to create casts between user-defined types and the standard string types — that's text, varchar, and char(n), plus any user-defined types that are defined to be in the string category. PostgreSQL already provides automatic I/O conversion casts for all of that, no cap. 🫶

So here's the tea on how those automatic ones behave:

  • The automatic casts to string types are treated as assignment casts.
  • The automatic casts from string types are explicit-only.

You can override this by declaring your own cast to replace an automatic one, but like… usually the only reason to do that is if you want the conversion to be more easily invokable than the standard assignment-only or explicit-only setting. That's it. That's the reason. 🎯

The other possible reason? You want the conversion to behave differently from the type's I/O function. But babe, that is sufficiently surprising that you should think twice about whether it's a good idea. 🙈 (A small number of the built-in types do indeed have different behaviors for conversions, mostly because of requirements of the SQL standard. So it happens! Just… be intentional.)

✨ the naming convention (lowkey iconic) ✨

While not required, it is recommended that you continue to follow this old convention of naming cast implementation functions after the target data type. Here's why, and it's a good reason:

Many users are used to being able to cast data types using a function-style notation, that is typename(x). And this notation is in fact nothing more nor less than a call of the cast implementation function — it is not specially treated as a cast. So if your conversion functions aren't named to support this convention, you will have surprised users. 😳 And not the fun kind of surprise.

The good news? Since PostgreSQL allows overloading of the same function name with different argument types, there is no difficulty in having multiple conversion functions from different types that all use the target type's name. Slay. 👑

Note 💀 Actually the preceding paragraph is an oversimplification: there are two cases in which a function-call construct will be treated as a cast request without having matched it to an actual function. 1. If a function call name(x) does not exactly match any existing function, but name is the name of a data type and pg_cast provides a binary-coercible cast to this type from the type of x, then the call will be construed as a binary-coercible cast. This exception is made so that binary-coercible casts can be invoked using functional syntax, even though they lack any function. 2. Likewise, if there is no pg_cast entry but the cast would be to or from a string type, the call will be construed as an I/O conversion cast. This exception allows I/O conversion casts to be invoked using functional syntax.
Note 🌀 There is also an exception to the exception: I/O conversion casts from composite types to string types cannot be invoked using functional syntax, but must be written in explicit cast syntax (either CAST or :: notation). This exception was added because after the introduction of automatically-provided I/O conversion casts, it was found too easy to accidentally invoke such a cast when a function or column reference was intended. So like… that's a whole trap, bestie. Watch your step. 🚧

🌸 EXAMPLES 🌸

Wanna create an assignment cast from type bigint to type int4 using the function int4(bigint)? Say less:

CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

(This cast is already predefined in the system.) So like, it's giving demo energy, not "you need to do this" energy. 💜

💖 COMPATIBILITY 💖

The CREATE CAST command conforms to the SQL standard, with a couple of caveats, because of course there are:

  • SQL does not make provisions for binary-coercible types or extra arguments to implementation functions.
  • AS IMPLICIT is a PostgreSQL extension, too.

So that's the standard being standard and Postgres being extra. As usual. 💅

👀 SEE ALSO 👀

That's the whole page, bestie — go forth and cast responsibly. 🦋✨ This one came to you straight from PostgreSQL 18.6, dated 2026, filed under CREATE CAST(7). Love that for us. 🎀💖

📦 where this page comes from