Index Onlys Scan for expressions

Hi, hackers!

There is a known issue that index only scan (IOS) can only work with
simple index keys based on single attributes and doesn't work with index
expressions. In this patch I propose a solution that adds support of IOS
for index expressions. Here's an example:

create table abc(a int, b int, c int);
create index on abc ((a * 1000 + b), c);

with t1 as (select generate_series(1, 1000) as x),
t2 as (select generate_series(0, 999) as x)
insert into abc(a, b, c)
select t1.x, t2.x, t2.x from t1, t2;
vacuum analyze;

Explain results with the patch:

explain (analyze, buffers) select a * 1000 + b + c from abc where a *
1000 + b = 1001;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Index Only Scan using abc_expr_c_idx on abc (cost=0.42..4.45 rows=1
width=4) (actual time=0.032..0.033 rows=1 loops=1)
Index Cond: ((((a * 1000) + b)) = 1001)
Heap Fetches: 0
Buffers: shared hit=4
Planning time: 0.184 ms
Execution time: 0.077 ms
(6 rows)

Before the patch it was:

explain (analyze, buffers) select a * 1000 + b + c from abc where a *
1000 + b = 1001;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Index Scan using abc_expr_c_idx on abc (cost=0.42..8.45 rows=1
width=4) (actual time=0.039..0.041 rows=1 loops=1)
Index Cond: (((a * 1000) + b) = 1001)
Buffers: shared hit=4
Planning time: 0.177 ms
Execution time: 0.088 ms
(5 rows)

This solution has limitations though: the restriction or the target
expression tree (or its part) must match exactly the index. E.g. this
expression will pass the check:

select a * 1000 + b + 100 from ...

but this will fail:

select 100 + a * 1000 + b from ...

because the parser groups it as:

(100 + a * 1000) + b

In this form it won't match any index key. Another case is when we
create index on (a+b) and then make query like 'select b+a ...' or '...
where b+a = smth' -- it won't match. This applies to regular index scan
too. Probably it worth to discuss the way to normalize index expressions
and clauses and work out more convenient way to match them.
Anyway, I will be grateful if you take a look at the patch in
attachment. Any comments and tips are welcome.

Thanks!

--
Ildar Musin
i(dot)musin(at)postgrespro(dot)ru