Traversing targetlist to find accessed columns

Hi,

Having a query, I am trying to find out all the columns that need to be
accessed (their varattno and vartype). I have access to a targetlist
representing a tree like this. So, I am looking for a function that
recursively traverses the tree and gives me the VARs. So, for SELECT
a,b,b+c from tab; I am interested in [a,b]. Is such a function currently
implemented in postgresql? How can I use it?

:targetlist (
> {TARGETENTRY
> :expr
> {VAR
> :varno 1
> :varattno 1
> :vartype 23
> :vartypmod -1
> :varcollid 0
> :varlevelsup 0
> :varnosyn 1
> :varattnosyn 1
> :location 7
> }
> :resno 1
> :resname l_orderkey
> :ressortgroupref 0
> :resorigtbl 24805
> :resorigcol 1
> :resjunk false
> }
> {TARGETENTRY
> :expr
> {VAR
> :varno 1
> :varattno 2
> :vartype 23
> :vartypmod -1
> :varcollid 0
> :varlevelsup 0
> :varnosyn 1
> :varattnosyn 2
> :location 18
> }
> :resno 2
> :resname l_partkey
> :ressortgroupref 0
> :resorigtbl 24805
> :resorigcol 2
> :resjunk false
> }
> {TARGETENTRY
> :expr
> {OPEXPR
> :opno 551
> :opfuncid 177
> :opresulttype 23
> :opretset false
> :opcollid 0
> :inputcollid 0
> :args (
> {OPEXPR
> :opno 551
> :opfuncid 177
> :opresulttype 23
> :opretset false
> :opcollid 0
> :inputcollid 0
> :args (
> {VAR
> :varno 1
> :varattno 1
> :vartype 23
> :vartypmod -1
> :varcollid 0
> :varlevelsup 0
> :varnosyn 1
> :varattnosyn 1
> :location 28
> }
> {VAR
> :varno 1
> :varattno 2
> :vartype 23
> :vartypmod -1
> :varcollid 0
> :varlevelsup 0
> :varnosyn 1
> :varattnosyn 2
> :location 39
> }
> )
> :location 38
> }
> {VAR
> :varno 1
> :varattno 3
> :vartype 23
> :vartypmod -1
> :varcollid 0
> :varlevelsup 0
> :varnosyn 1
> :varattnosyn 3
> :location 49
> }
> )
> :location 48
> }
> :resno 3
> :resname ?column?
> :ressortgroupref 0
> :resorigtbl 0
> :resorigcol 0
> :resjunk false
> }
> )
>