Re: New CORRESPONDING clause design

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Surafel Temesgen <surafel3000(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New CORRESPONDING clause design
Date: 2017-04-01 17:54:45
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Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> [ corresponding_clause_v12.patch ]

I worked on this for awhile but eventually decided that it's not very
close to being committable. The main thing that's scaring me off is
a realization that there are a *lot* of places that assume that the
output columns of a set operation are one-for-one with the columns of
the inputs. One such place is the subquery qual pushdown logic in
allpaths.c, which this patch hasn't touched, resulting in

regression=# create table t1 (a int, b int, c int);
regression=# create table t2 (a int, b int, c int);
regression=# create view vv2 as
regression-# select * from t1 union all corresponding by (b,c) select * from t2;
regression=# select * from vv2 where c = 44;
ERROR: wrong number of tlist entries

Another is the code that converts a UNION ALL subquery into an appendrel.
The best thing there might be to just reject CORRESPONDING in
is_simple_union_all, but this patch doesn't, which means we get the wrong
results from

regression=# create table t3 (b int, a int, c int);
regression=# explain verbose select * from t1 union all corresponding select * from t3;
Append (cost=0.00..60.80 rows=4080 width=12)
-> Seq Scan on public.t1 (cost=0.00..30.40 rows=2040 width=12)
Output: t1.a, t1.b, t1.c
-> Seq Scan on public.t3 (cost=0.00..30.40 rows=2040 width=12)
Output: t3.b, t3.a, t3.c
(5 rows)

Notice it's failed to rearrange the columns to match.

There are also such assumptions in ruleutils.c. Trying to reverse-list
the above view gives

regression=# \d+ vv2
View "public.vv2"
Column | Type | Collation | Nullable | Default | Storage | Description
b | integer | | | | plain |
c | integer | | | | plain |
View definition:
SELECT t1.a AS b,
t1.b AS c,
SELECT t2.a AS b,
t2.b AS c,
FROM t2;

which is obviously wrong. The reason for that is that the code is
trying to ensure that the SELECT's output column names match the
view's column names, so it sticks AS clauses on the select-list
expressions. If we go a little further:

regression=# alter table vv2 rename column c to ceetwo;
regression=# \d+ vv2
View "public.vv2"
Column | Type | Collation | Nullable | Default | Storage | Description
b | integer | | | | plain |
ceetwo | integer | | | | plain |
View definition:
SELECT t1.a AS b,
t1.b AS ceetwo,
SELECT t2.a AS b,
t2.b AS ceetwo,
FROM t2;

Now things are a *complete* mess, because this view definition would
successfully parse during a dump-and-reload, but it means something
else than it did before; the CORRESPONDING BY list has failed to track
the change in names of the columns.

In general, there's a lot of subtle logic in ruleutils.c about what we
need to do to ensure that reverse-listed views keep the same semantic
meaning in the face of column renamings or additions in their input
tables. CORRESPONDING is really scary in this situation because its
semantics depend on column names. I tried to break it by adding/renaming
columns of the input tables to see if I could get ruleutils to print
something that didn't mean what it meant before. I didn't succeed
immediately, but I am thinking it would be smart for us always to
reverse-list the construct as CORRESPONDING BY with an explicit list of
column names. We don't ever reverse-list a NATURAL JOIN as such,
preferring to use an explicit JOIN USING list, for closely related
reasons. (You might care to study the logic in ruleutils.c around the
usingNames list, which exists to ensure that JOIN USING doesn't get broken
by column renames. It's entirely likely that we're going to need
something equivalently complicated for CORRESPONDING BY. Or if we
don't, I'd want to have some comments in there that clearly explain
why we don't, because otherwise somebody will break it in future.)

I also found that WITH RECURSIVE fails immediately if you stick
CORRESPONDING into the recursive union step; eg in this lightly
modified version of a query from with.sql,

parent_id INTEGER REFERENCES tree(id)

WITH RECURSIVE t(id, path) AS (
select 1 as id, ARRAY[]::integer[] as path
SELECT, t.path || as path
FROM tree JOIN t ON (tree.parent_id =
SELECT t1.*, t2.* FROM t AS t1 JOIN t AS t2 ON
(t1.path[1] = t2.path[1] AND
array_upper(t1.path,1) = 1 AND
array_upper(t2.path,1) > 1)

ERROR: column does not exist
LINE 5: FROM tree JOIN t ON (tree.parent_id =
HINT: Perhaps you meant to reference the column "".

I was expecting to find trouble if the CORRESPONDING had to rearrange
columns (breaking the 1-to-1 assumption), but it fails even without that.

There may well be other places in the system that are making similar
assumptions about 1-to-1 mapping between setop inputs and outputs.
This comment in plan_set_operations is pretty scary, for instance:
* Find the leftmost component Query. We need to use its column names for
* all generated tlists (else SELECT INTO won't work right).
because the column names in the component query certainly won't be
1-to-1 with upper tlists if CORRESPONDING is active.

I wonder whether it wouldn't be a smarter plan to abandon this
implementation approach and rely on inserting an explicit level of
sub-selects during the parse analysis transformation. That is,


would get converted into

SELECT x, y FROM (SELECT * FROM foo) ss1
SELECT x, y FROM (SELECT * FROM bar) ss2

This isn't very pretty, but I would have a lot fewer worries
about how long we'd be finding bugs induced by the feature.

If we do stick with trying to represent CORRESPONDING BY explicitly
in the parse tree, I'm not very satisfied with the representation
you've chosen for SetOperationStmt.correspondingColumns. The patch

+ List *correspondingColumns; /* list of corresponding column names */

which is a flat out lie. debug_print_parse and some study of the code
showed me that it's actually a list of TargetEntry nodes containing
copies of the left-hand input's target expression trees. This seems
bulky, bizarre, and asymmetric; among other things it requires the planner
to re-derive the column matching that the parser had already figured out.
There certainly doesn't seem to be any value in carrying around an extra
copy of the left-hand expressions.

I'd be inclined to suggest that the best representation in
SetOperationStmt is two integer lists of the left-hand and right-hand
column numbers of the CORRESPONDING columns. This would probably
simplify matters greatly for the planner. It would mean that ruleutils.c
would have to do a bit more work to find out the column names to print
in CORRESPONDING BY; but as I showed above, just printing the names that
appeared at parse time is a doomed strategy anyway.

Also, just in passing --- while it may well be a good idea to add a
location field to struct Value, I'm not on board with doing so and
then using it in only one place. As I said earlier, I think that
CORRESPONDING's column name list should act like every other column
name list in the grammar, and that includes making use of error cursor
locations where appropriate. And changing struct Value has repercussions
way beyond that, for instance it's not clear why we'd keep A_Const as a
separate node type if Value has a location field. I think if you want
to push that forward, it would be a good idea to submit a separate
patch that just focuses on adding error cursor location reports
everywhere that adding locations to Values would enable.

I'll set this back to Waiting on Author, but I think the chances of
getting to a committable patch before the end of the commitfest are
about nil.

regards, tom lane

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