Hello again everyone!
I've been working on a prototype
<https://issues.apache.org/jira/browse/CASSANDRA-17719> in CASSANDRA-17719
for a grammar that roughly corresponds to what we've agreed on in this
thread. One thing that isn't immediately obvious to me is how the LET
syntax handles cases where we want to check for the plain existence of a
row in IF. For example, in this hybrid of the originally proposed syntax
and something more like what we've agreed on (and the RETURNING just to
distinguish between that and SELECT), this could be pretty straightforward:
BEGIN TRANSACTION
SELECT v FROM ks.tbl WHERE k=0 AND c=0 AS row1;
SELECT v FROM ks.tbl WHERE k=1 AND c=0 AS row2;
RETURNING row1.v, row2.v
IF row1 NOT EXISTS AND row2.v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
The NOT EXISTS operator has row1 to work with. One the other hand, w/ the
LET syntax and no naming of reads, it's not clear what the best solution
would be. Here are a few possibilities:
1.) Provide a few built-in functions that operate on a whole result row. If
we assume a SQL style IS NULL and IS NOT NULL (see my last post here) for
operations on particular columns, this probably eliminates the need for
EXISTS/NOT EXISTS as well.
BEGIN TRANSACTION
LET row1_missing = notExists() FROM ks.tbl WHERE k=0 AND c=0;
LET row2_v = v FROM ks.tbl WHERE k=1 AND c=0;
SELECT row1_missing, row2_v
IF row1_missing AND row2_v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
2.) Assign and check the first primary key element to determine whether the
row exists.
BEGIN TRANSACTION
LET row1_k = k FROM ks.tbl WHERE k=0 AND c=0;
LET row2_v = v FROM ks.tbl WHERE k=1 AND c=0;
SELECT row1_k, row2_v
IF row1_k IS NULL AND row2_v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
3.) Reconsider the LET concept toward something that allows us to
explicitly name our reads again.
BEGIN TRANSACTION
WITH (SELECT v FROM ks.tbl WHERE k=0 AND c=0) AS row1;
WITH (SELECT v FROM ks.tbl WHERE k=1 AND c=0) AS row2;
SELECT row1.v, row2.v
IF row1 NOT EXISTS AND row2.v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
I don't have a strong affinity for any of these, although #1 seems the most
awkward.
Does anyone have any other alternatives? Preference for one of the above
options?
Thanks!
On Fri, Jul 22, 2022 at 11:21 AM Caleb Rackliffe <calebrackli...@gmail.com>
wrote:
> Avi brought up an interesting point around NULLness checking in
> CASSANDRA-17762 <https://issues.apache.org/jira/browse/CASSANDRA-17762>...
>
> In SQL, any comparison with NULL is NULL, which is interpreted as FALSE in
>> a condition. To test for NULLness, you use IS NULL or IS NOT NULL. But LWT
>> uses IF col = NULL as a NULLness test. This is likely to confuse people
>> coming from SQL and hamper attempts to extend the dialect.
>
>
> We can leave that Jira open to address what to do in the legacy LWT case,
> but I'd support a SQL-congruent syntax here (IS NULL or IS NOT NULL),
> where we have something closer to a blank slate.
>
> Thoughts?
>
> On Thu, Jun 30, 2022 at 6:25 PM Abe Ratnofsky <a...@aber.io> wrote:
>
>> The new syntax looks great, and I’m really excited to see this coming
>> together.
>>
>> One piece of feedback on the proposed syntax is around the use of “=“ as
>> a declaration in addition to its current use as an equality operator in a
>> WHERE clause and an assignment operator in an UPDATE:
>>
>> BEGIN TRANSACTION
>> LET car_miles = miles_driven, car_is_running = is_running FROM cars
>> WHERE model=’pinto’
>> LET user_miles = miles_driven FROM users WHERE name=’blake’
>> SELECT something else from some other table
>> IF NOT car_is_running THEN ABORT
>> UPDATE users SET miles_driven = user_miles + 30 WHERE name='blake';
>> UPDATE cars SET miles_driven = car_miles + 30 WHERE model='pinto';
>> COMMIT TRANSACTION
>>
>> This is supported in languages like PL/pgSQL, but in a normal SQL query
>> kind of local declaration is often expressed as an alias (SELECT col AS
>> new_col), subquery alias (SELECT col) t, or common table expression (WITH t
>> AS (SELECT col)).
>>
>> Here’s an example of an alternative to the proposed syntax that I’d find
>> more readable:
>>
>> BEGIN TRANSACTION
>> WITH car_miles, car_is_running AS (SELECT miles_driven, is_running FROM
>> cars WHERE model=’pinto’),
>> user_miles AS (SELECT miles_driven FROM users WHERE name=’blake’)
>> IF NOT car_is_running THEN ABORT
>> UPDATE users SET miles_driven = user_miles + 30 WHERE name='blake';
>> UPDATE cars SET miles_driven = car_miles + 30 WHERE model='pinto';
>> COMMIT TRANSACTION
>>
>> There’s also the option of naming the transaction like a subquery, and
>> supporting LET via AS (this one I’m less sure about but wanted to propose
>> anyway):
>>
>> BEGIN TRANSACTION t1
>> SELECT miles_driven AS t1.car_miles, is_running AS t1.car_is_running
>> FROM cars WHERE model=’pinto’;
>> SELECT miles_driven AS t1.user_miles FROM users WHERE name=’blake’;
>> IF NOT car_is_running THEN ABORT
>> UPDATE users SET miles_driven = user_miles + 30 WHERE name='blake';
>> UPDATE cars SET miles_driven = car_miles + 30 WHERE model='pinto';
>> COMMIT TRANSACTION
>>
>> This also has the benefit of resolving ambiguity in case of naming
>> conflicts with existing (or future) column names.
>>
>> --
>> Abe
>>
>
