Hi, everybody!
The major reason why we have so small number of GiST/SP-GiST/GIN implementations is that their interface function takes arguments of "internal" datatype (actually they are mostly pointers, not normal PostgreSQL datatypes). So, you actually can implement them in C. It discourages many users.
Idea of project is to provide alternative interfaces for GiST/SP-GiST/GIN which deal with composite types and arrays. Then users can write GiST/SP-GiST/GIN implementations on whatever they like SQL, pl/pgSQL, pl/Python etc.
Probably someone would like to put his hands on it?
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With best regards,
Alexander Korotkov.