plpgsql keywords are hidden reserved words

I was somewhat bemused just now to find that this function stopped
working:

regression=# create function estimate_rows(query text) returns float8 as $$
declare r text;
begin
for r in execute 'explain ' || query loop
if substring(r from 'rows=[0-9]') is not null then
return substring (r from 'rows=([0-9]+)');
end if;
end loop;
return null;
end$$ language plpgsql strict;
CREATE FUNCTION
regression=# select estimate_rows('select * from tenk1 where unique1<500');
ERROR: column "query" does not exist
LINE 1: SELECT 'explain ' || query
^
QUERY: SELECT 'explain ' || query
CONTEXT: PL/pgSQL function "estimate_rows" line 3 at FOR over EXECUTE statement

This works fine in 8.2. The reason it no longer works is that "query"
is now a special token in the plpgsql lexer, and that means that it will
never be substituted for by read_sql_construct(). So it's effectively
a reserved word.

While I can work around this by changing the parameter name or using

for r in execute 'explain ' || estimate_rows.query loop

it's still a tad annoying, and it means that we have to be *very*
circumspect about adding new keywords to plpgsql.

I don't see any fix for this that's reasonable to try to shoehorn
into 8.3, but I think we really need to revisit the whole area of
plpgsql variable substitution during 8.4. We could make this problem
go away if variable substitution happened through a parser callback
instead of before parsing.

regards, tom lane