insert with multiple targetLists

Hi all,

I've been taking a look at fixing the TODO item:

o Allow INSERT INTO tab (col1, ..) VALUES (val1, ..), (val2, ..)

My first plan of attack was to replace the current list of ResTargets
in InsertStmt with a list of lists. The problem with that approach is
that:

(a) the InsertStmt is converted to a Query. I could also change Query
to use a list of lists (instead of a list) for holding TargetEntry
items, but that would be ugly (since Query is generic, and this
would only be needed for Inserts)

(b) modifying Query would mean a lot of work (e.g. in the rewriter),
adapting all the places that expect targetList to be a list to
instead use a list of lists. Once again, this would be messy.

So, that seems like a bad idea.

ISTM that a better way to do this would be to parse the InsertStmt,
and then execute an INSERT for every targetList in the query.
For example:

INSERT INTO t1 (c1) VALUES (1), (2);

would be executed in a similar fashion to:

INSERT INTO t1 (c1) VALUES (1);
INSERT INTO t1 (c1) VALUES (2);

Does this sound reasonable?

Any suggestions would be welcome.

Cheers,

Neil

--
Neil Conway <neilconway(at)rogers(dot)com>
PGP Key ID: DB3C29FC

> INSERT INTO t1 (c1) VALUES (1), (2);
>
> would be executed in a similar fashion to:
>
> INSERT INTO t1 (c1) VALUES (1);
> INSERT INTO t1 (c1) VALUES (2);
>
> Does this sound reasonable?

I debated doing the above too. In fact, I had a partial
implementation at one point.

However, the resulting purpose of allowing such a construct is to
enable the speeds copy achieves with the variation that is found in an
insert. So, the above transformation method really doesn't accomplish
much except a new style for many inserts. But it is quite a bit
easier simply to code each insert individually if there is a minimal
speed gain. Large strings may reach query length limits in other
systems using this style (look at a MySQL dump sometime). You're
really only good for about 50 or 60 records in a single insert
statement there.

I'd tend to run it like a copy that can resolving expressions and
defaults.

Neil Conway <nconway(at)klamath(dot)dyndns(dot)org> writes:
> I've been taking a look at fixing the TODO item:
> o Allow INSERT INTO tab (col1, ..) VALUES (val1, ..), (val2, ..)
> My first plan of attack was to replace the current list of ResTargets
> in InsertStmt with a list of lists.

If you look at the SQL spec, they actually consider VALUES to be a
<table value constructor> which is one of the base cases for <query
expression>. Thus for example this is legal SQL (copied and pasted
straight from the spec):

CONSTRAINT VIEWS_IS_UPDATABLE_CHECK_OPTION_CHECK
CHECK ( ( IS_UPDATABLE, CHECK_OPTION ) NOT IN
( VALUES ( 'NO', 'CASCADED' ), ( 'NO', 'LOCAL' ) ) )

So one should really think of INSERT...VALUES as a form of
INSERT...SELECT rather than a special case of its own. INSERT...SELECT
is currently extremely klugy (look at the hacks in the rewriter for it)
so I think you will not get very far until you redesign the querytree
structure for INSERT...SELECT.

BTW, all the non-trivial cases for VALUES are Full SQL only, not entry
or even intermediate level. So I don't see much point in providing a
half-baked implementation. We've still got lots left to do to cover
all of the entry-SQL spec, and IMHO we ought to focus on that stuff
first...

regards, tom lane

Rod Taylor wrote:
> > INSERT INTO t1 (c1) VALUES (1), (2);
> >
> > would be executed in a similar fashion to:
> >
> > INSERT INTO t1 (c1) VALUES (1);
> > INSERT INTO t1 (c1) VALUES (2);
> >
> > Does this sound reasonable?

Sounds good to me.

> I debated doing the above too. In fact, I had a partial
> implementation at one point.
>
> However, the resulting purpose of allowing such a construct is to
> enable the speeds copy achieves with the variation that is found in an
> insert. ...

I thought the purpose of the item was merely for compatibility with
other databases that support this syntax. I don't think it will ever
match COPY performance, and I don't think stuffing a huge INSERT into
the database rather than COPY rows will ever be a preferred method.

I only see VALUES used by INSERT so if you can think of a clean way to
make that work as multiple INSERTs, I think it would be a good idea.
Hopefully, it will be one localized change, and we can remove it if we
ever want to support VALUES in more complex situations, as Tom
mentioned.

--
Bruce Momjian | http://candle.pha.pa.us
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