PostgreSQL 型別系統包含許多專用項目，這些項目統稱為概念型別(pseudo-type)。概念型別不能用作實際的欄位資料型別，但可以用來宣告函數的參數或結果型別。各個概念型別在於函數的行為不只是對應於簡單地獲取或回傳特定 SQL 資料型別的值的情況下很有用。Table 8.27 列出了現有的概念型別。
Functions coded in C (whether built-in or dynamically loaded) can be declared to accept or return any of these pseudo data types. It is up to the function author to ensure that the function will behave safely when a pseudo-type is used as an argument type.
Functions coded in procedural languages can use pseudo-types only as allowed by their implementation languages. At present most procedural languages forbid use of a pseudo-type as an argument type, and allow only
recordas a result type (plus
event_triggerwhen the function is used as a trigger or event trigger). Some also support polymorphic functions using the types
internalpseudo-type is used to declare functions that are meant only to be called internally by the database system, and not by direct invocation in an SQL query. If a function has at least one
internal-type argument then it cannot be called from SQL. To preserve the type safety of this restriction it is important to follow this coding rule: do not create any function that is declared to return
internalunless it has at least one