postgresql-server-dev-12 12.5-0ubuntu0.20.04.1 (amd64 binary) in ubuntu focal

 Header files for compiling SSI code to link into PostgreSQL's backend; for
 example, for C functions to be called from SQL.
 .
 This package also contains the Makefiles necessary for building add-on
 modules of PostgreSQL, which would otherwise have to be built in the
 PostgreSQL source-code tree.
 .
 PostgreSQL is an object-relational SQL database management system.