On 12/24/2013 03:21 PM, Abbas Butt wrote:

> Could you please explain a little bit more how would you solve the posed
> problem using map_variable_attnos?
> I was recently working on a similar problem and used the following algo
> to solve it.
> I had to find to which column of the base table does a column in
> the select statement of the view query belong.
> To relate a target list entry in the select query of the view to
> an actual column in base table.

Sounds similar. My problem is simplified by the constraint that the view
must be a simple view (only one base relation) and must only contain
simple column-references to single the base relation. No  expressions,
no joins. (These are the rules for simply updatable views).

I'm new to the planner and rewriter, so don't take what I do as any kind
of example beyond "this seems to work".

I generate a varattno mapping as follows:

 * Scan the passed view target list, whose members must consist solely
 * of Var nodes with a varno equal to the passed targetvarno.
 * A mapping is built from the resno (i.e. tlist index) of the view
 * tlist to the corresponding attribute number of the base relation
 * the varattno points to.
 * Must not be called with a targetlist containing non-Var entries.
static void
gen_view_base_attr_map(List *viewtlist, AttrNumber * attnomap, int
    ListCell    *lc;
    TargetEntry *te;
    Var         *tev;
    int         l_viewtlist = list_length(viewtlist);

    foreach(lc, viewtlist)
        te = (TargetEntry*) lfirst(lc);
        /* Could relax this in future and map only the var entries,
         * ignoring everything else, but currently pointless since we
         * are only interested in simple views. */
        Assert(IsA(te->expr, Var));
        tev = (Var*) te->expr;
        Assert(tev->varno == targetvarno);
        Assert(te->resno - 1 < l_viewtlist);
        attnomap[te->resno - 1] = tev->varattno;

producing a forward mapping of view attno to base relation attno.

I then apply the varattno remapping, in this case to the returning list
of a DML query acting on a view, with something like:

    varattno_map = palloc( list_length(viewquery->targetList) *
                           sizeof(AttrNumber) );

                           varattno_map, rtr->rtindex);

    parsetree->returningList = map_variable_attnos(
            (Node*) parsetree->returningList,
            old_result_rt_index, 0,
            varattno_map, list_length(viewquery->targetList),

    if (found_whole_row_var)
        /* TODO: Extend map_variable_attnos API to
           pass a mutator to handle whole-row vars. */
        elog(ERROR, "RETURNING list contains a whole-row variable, "
                    "which is not currently supported for updatable "

    ChangeVarNodes((Node*) parsetree->returningList,

I'd prefer to be doing the map_variable_attnos and ChangeVarNodes work
in a single pass, but it looks like a clumsy and verbose process to
write a new walker. So I'm going to leave it as an "opportunity for
future optimisation" for now ;-)

(As it happens that "found_whole_row_var" is a real pain; I'm going to
have to deal with a TODO item in map_variable_attnos to provide a
callback that replaces a whole-row Var with an expansion of it into a

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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