The package being used by postgresql.
Declarations
Type
packageDefault
if versionAtLeast config.system.stateVersion "25.11" then
  pkgs.postgresql_17
else if versionAtLeast config.system.stateVersion "24.11" then
  pkgs.postgresql_16
else if versionAtLeast config.system.stateVersion "23.11" then
  pkgs.postgresql_15
else if versionAtLeast config.system.stateVersion "22.05" then
  pkgs.postgresql_14
else
  pkgs.postgresql_13
Example
pkgs.postgresql_15