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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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 in PL/Tcl, but there are some restrictions that must be understood.
For security reasons, PL/Tcl executes functions called by any one SQL role in a separate Tcl interpreter for that role. This prevents accidental or malicious interference by one user with the behavior of another user's PL/Tcl functions. Each such interpreter will have its own values for any "global" Tcl variables. Thus, two PL/Tcl functions will share the same global variables if and only if they are executed by the same SQL role. In an application wherein a single session executes code under multiple SQL roles (via SECURITY DEFINER functions, use of SET ROLE, etc) you may need to take explicit steps to ensure that PL/Tcl functions can share data. To do that, make sure that functions that should communicate are owned by the same user, and mark them SECURITY DEFINER. You must of course take care that such functions can't be used to do anything unintended.
All PL/TclU functions used in a session execute in the same Tcl interpreter, which of course is distinct from the interpreter(s) used for PL/Tcl functions. So global data is automatically shared between PL/TclU functions. This is not considered a security risk because all PL/TclU functions execute at the same trust level, namely that of a database superuser.
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 persistent private data of a function. Use regular Tcl global
variables only for values that you specifically intend to be shared among
multiple functions. (Note that the GD arrays are only
global within a particular interpreter, so they do not bypass the
security restrictions mentioned above.)
An example of using GD appears in the
spi_execp example below.