## 36.4. Global Data in PL/Tcl

Sometimes it is useful to have some global data that is held between two calls to a function or is shared between different functions. This is easily done since all PL/Tcl functions executed in one session share the same safe Tcl interpreter. So, any global Tcl variable is accessible to all PL/Tcl function calls and will persist for the duration of the SQL session. (Note that PL/TclU functions likewise share global data, but they are in a different Tcl interpreter and cannot communicate with PL/Tcl functions.)

To help protect PL/Tcl functions from unintentionally interfering with each other, a global array is made available to each function via the `upvar` command. The global name of this variable is the function's internal name, and the local name is `GD`. It is recommended that `GD` be used for private data of a function. Use regular Tcl global variables only for values that you specifically intend to be shared among multiple functions.

An example of using `GD` appears in the `spi_execp` example below.