POSIX

This SQL standard collation sorts using the Unicode Collation Algorithm with the Default Unicode Collation Element Table. It is available in all encodings. ICU support is required to use this collation, and behavior may change if PostgreSQL is built with a different version of ICU. (This collation has the same behavior as the ICU root locale; see und-x-icu (for “undefined”).)

This SQL standard collation sorts using the Unicode code point values rather than natural language order, and only the ASCII letters “A” through “Z” are treated as letters. The behavior is efficient and stable across all versions. Only available for encoding UTF8. (This collation has the same behavior as the libc locale specification C in UTF8 encoding.)

This collation sorts by Unicode code point values rather than natural language order. For the functions lower, initcap, and upper it uses Unicode full case mapping. For pattern matching (including regular expressions), it uses the Standard variant of Unicode Compatibility Properties. Behavior is efficient and stable within a Postgres major version. It is only available for encoding UTF8.

This collation sorts by Unicode code point values rather than natural language order. For the functions lower, initcap, and upper, it uses Unicode simple case mapping. For pattern matching (including regular expressions), it uses the POSIX Compatible variant of Unicode Compatibility Properties. Behavior is efficient and stable within a PostgreSQL major version. This collation is only available for encoding UTF8.

The C and POSIX collations are based on “traditional C” behavior. They sort by byte values rather than natural language order, and only the ASCII letters “A” through “Z” are treated as letters. The behavior is efficient and stable across all versions for a given database encoding, but behavior may vary between different database encodings.

The default collation selects the locale specified at database creation time.