In commit 1bc16a94 I added a minor
optimization to drop the component variables of a GROUP BY expression from
the target list computed at the aggregation level of a query, if those Vars
weren't referenced elsewhere in the tlist.  However, I overlooked that the
window-function planning code would deconstruct such expressions and thus
need to have access to their component variables.  Fix it to not do that.

While at it, I removed the distinction between volatile and nonvolatile
window partition/order expressions: the code now computes all of them
at the aggregation level.  This saves a relatively expensive check for
volatility, and it's unclear that the resulting plan isn't better anyway.

Per bug #7535 from Louis-David Mitterrand.  Back-patch to 9.2.

Add a column pg_class.relallvisible to remember the number of pages that
were all-visible according to the visibility map as of the last VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to estimate
how many heap page fetches can be avoided during an index-only scan.

This is pretty primitive and will no doubt see refinements once we've
acquired more field experience with the index-only scan mechanism, but
it's way better than using a constant.

Note: I had to adjust an underspecified query in the window.sql regression
test, because it was changing answers when the plan changed to use an
index-only scan.  Some of the adjacent tests perhaps should be adjusted
as well, but I didn't do that here.

In commit c1d9579d, I changed things so
that the output of the Agg node that feeds the window functions would not
list any ungrouped Vars directly.  Formerly, for example, the Agg tlist
might have included both "x" and "sum(x)", which is not really valid if
"x" isn't a grouping column.  If we then had a window function ordering on
something like "sum(x) + 1", prepare_sort_from_pathkeys would find no exact
match for this in the Agg tlist, and would conclude that it must recompute
the expression.  But it would break the expression down to just the Var
"x", which it would find in the tlist, and then rebuild the ORDER BY
expression using a reference to the subplan's "x" output.  Now, after the
above-referenced changes, "x" isn't in the Agg tlist if it's not a grouping
column, so that prepare_sort_from_pathkeys fails with "could not find
pathkey item to sort", as reported by Bricklen Anderson.

The fix is to not break down Aggrefs into their component parts, but just
treat them as irreducible expressions to be sought in the subplan tlist.
This is definitely OK for the use with respect to window functions in
grouping_planner, since it just built the tlist being used on the same
basis.  AFAICT it is safe for other uses too; most of the other call sites
couldn't encounter Aggrefs anyway.

Regular aggregate functions in combination with, or within the arguments
of, window functions are OK per spec; they have the semantics that the
aggregate output rows are computed and then we run the window functions
over that row set.  (Thus, this combination is not really useful unless
there's a GROUP BY so that more than one aggregate output row is possible.)
The case without GROUP BY could fail, as recently reported by Jeff Davis,
because sloppy construction of the Agg node's targetlist resulted in extra
references to possibly-ungrouped Vars appearing outside the aggregate
function calls themselves.  See the added regression test case for an
example.

Fixing this requires modifying the API of flatten_tlist and its underlying
function pull_var_clause.  I chose to make pull_var_clause's API for
aggregates identical to what it was already doing for placeholders, since
the useful behaviors turn out to be the same (error, report node as-is, or
recurse into it).  I also tightened the error checking in this area a bit:
if it was ever valid to see an uplevel Var, Aggref, or PlaceHolderVar here,
that was a long time ago, so complain instead of ignoring them.

Backpatch into 9.1.  The failure exists in 8.4 and 9.0 as well, but seeing
that it only occurs in a basically-useless corner case, it doesn't seem
worth the risks of changing a function API in a minor release.  There might
be third-party code using pull_var_clause.

This patch allows the frame to start from CURRENT ROW (in either RANGE or
ROWS mode), and it also adds support for ROWS n PRECEDING and ROWS n FOLLOWING
start and end points.  (RANGE value PRECEDING/FOLLOWING isn't there yet ---
the grammar works, but that's all.)

Hitoshi Harada, reviewed by Pavel Stehule

so that their elements are always taken as simple expressions over the
query's input columns.  It originally seemed like a good idea to make them
act exactly like GROUP BY and ORDER BY, right down to the SQL92-era behavior
of accepting output column names or numbers.  However, that was not such a
great idea, for two reasons:

1. It permits circular references, as exhibited in bug #5018: the output
column could be the one containing the window function itself.  (We actually
had a regression test case illustrating this, but nobody thought twice about
how confusing that would be.)

2. It doesn't seem like a good idea for, eg, "lead(foo) OVER (ORDER BY foo)"
to potentially use two completely different meanings for "foo".

Accordingly, narrow down the behavior of window clauses to use only the
SQL99-compliant interpretation that the expressions are simple expressions.

patch.  This includes the ability to force the frame to cover the whole
partition, and the ability to make the frame end exactly on the current row
rather than its last ORDER BY peer.  Supporting any more of the full SQL
frame-clause syntax will require nontrivial hacking on the window aggregate
code, so it'll have to wait for 8.5 or beyond.

results due to underspecified ordering.  Per report from buildfarm member
pika.

Hitoshi Harada, with some kibitzing from Heikki and Tom.