Preventing some SQL commands

In a PL language it's sometimes desirable to prevent execution of some
commands. I would like to prevent the commands "begin [work or
transaction]", "commit", and "rollback", completely and I would like to
force the user to use explicit methods for the savepoint methods.

I wonder if there's any way to extract the nature of a command from the
execution plan returned by SPI_prepare. If not, would it be very
difficult to add? It doesn't feel optimal to add a home brewed parser
that parses the statements prior to prepare just to find out if I they
should prevented.

One approach could be to extend the CmdType enum. Perhaps something like
this:

typedef enum CmdType
{
CMD_UNKNOWN,
CMD_SELECT, /* select stmt (formerly retrieve) */
CMD_UPDATE, /* update stmt (formerly replace) */
CMD_INSERT, /* insert stmt (formerly append) */
CMD_DELETE,
CMD_TRANSACTION, /* begin, commit, rollback */
CMD_SAVEPOINT, /* savepoint, rollback to savepoint, release
savepoint */
CMD_UTILITY, /* cmds like create, destroy, copy,
* vacuum, etc. */
CMD_NOTHING /* dummy command for instead nothing
rules
* with qual */
} CmdType;

and then add a SPI function

CmdType SPI_get_command_type(void* executionPlan)

What do you think? It would certanly help PL/Java add safe and efficient
savepoint management and the other PL's are likely to share my concerns.

Regards,
Thomas Hallgren