Ok it is possible, I've got a C extension up and running which hooks ExecutorStart, then once for each TX ( I monitor the nesting depth like in pg_stat_statements, and only attach at the top level) attaches a sub transaction callback, tracking start subtransaction events and incrementing a counter / keeping track of the worst offenders in a hashmap.
Sorry that should be ExecutorRun - although I'm a little confused about which is the correct choice.
-James
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