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CHDB (constant hash database) is a fast key-value database for constant data, realized by using a memory-mapped file and thus providing the following functionalities:

• Extremely fast initial load, regardless of the size of the database.

• Only the pages of the file which are actually used are loaded from the disk.

• Once a page is loaded it is shared across multiple processes.

• Loaded pages are cached across multiple requests and even process recycling.

A typical use of CHDB is as a faster alternative to defining many PHP constants.

CHDB is internally implemented as a hash-table using a » perfect hashing function, thus guaranteeing worst case O(1) lookup time.