TEXT vs PG_NODE_TREE in system columns (cross column and expression statistics patch)

Hi,

attached is the WIP patch for cross-column statistics and
extra expression statistics.

My question is that why pg_node_tree is unusable as
syscache attribute? I attempted to alias it as text in the patch
but I get the following error if I try to use it by setting
USE_SYSCACHE_FOR_SEARCH to 1 in selfuncs.c.
Directly using the underlying pg_statistic3 doesn't cause an error.

zozo=# select * from t1 where i+1 = 5;
ERROR:  could not determine which collation to use for string comparison
HINT:  Use the COLLATE clause to set the collation explicitly.

The table looks like this:

create table t1 (id serial primary key, t text, i integer);

Best regards,
Zoltán Böszörményi

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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt, Austria
Web: http://www.postgresql-support.de
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