What does :: do in PostgreSQL?
A type cast specifies a conversion from one data type to another.
PostgreSQL accepts two equivalent syntaxes for type casts, the PostgreSQL-specific value::type and the SQL-standard CAST(value AS type).
In this specific case, '{apple,cherry apple, avocado}'::text[]; takes the string literal {apple,cherry apple, avocado} and tells PostgreSQL to interpret it as an array of text.
See the documentation on SQL expressions and arrays for details.
What @PSR and @Craig wrote.
Plus, there are two more syntax variants:
type value
This form only casts constants (string literals). Like in:
SELECT date '2013-03-21';
More in the manual in the chapter Constants of Other Types.
2.type(value)
That's the function-like syntax. Works only for types whose names are valid as function names. Like in:
SELECT date(date_as_text_col) FROM tbl;
More in the manual in the chapter Type Casts.
More comprehensive answer:
- Postgres data type cast