On 13 Aug 2022, at 16:59, Patrick McFadin <pmcfa...@gmail.com> wrote:
I'm really happy to see CEP-15 getting closer to a final
implementation. I'm going to walk through my reasoning for your
proposals wrt trying to explain this to somebody new.
Looking at all the options, the first thing that comes up for me is
the Cassandra project's complicated relationship with NULL. We
have prior art with EXISTS/NOT EXISTS when creating new tables. IS
NULL/IS NOT NULL is used in materialized views similarly to
proposals 2,4 and 5.
CREATE MATERIALIZED VIEW [ IF NOT EXISTS ] [keyspace_name.]view_name
AS SELECT [ (column_list) ]
FROM [keyspace_name.]table_name
[ WHERE column_name IS NOT NULL
[ AND column_name IS NOT NULL ... ] ]
[ AND relation [ AND ... ] ]
PRIMARY KEY ( column_list )
[ WITH [ table_properties ]
[ [ AND ] CLUSTERING ORDER BY (cluster_column_name order_option)
] ] ;
Based on that, I believe 1 and 3 would just confuse users, so -1
on those.
Trying to explain the difference between row and column operations
with LET, I can't see the difference between a row and column in #2.
#4 introduces a boolean instead of column names and just adds more
syntax.
#5 is verbose and, in my opinion, easier to reason when writing a
query. Thinking top down, I need to know if these exact rows and/or
column values exist before changing them, so I'll define them
first. Then I'll iterate over the state I created in my actual
changes so I know I'm changing precisely what I want.
#5 could use a bit more to be clearer to somebody who doesn't write
CQL queries daily and wouldn't require memorizing subtle
differences. It should be similar to all the other syntax, so
learning a little about CQL will let you move into more without
completely re-learning the new syntax.
So I propose #6)
BEGIN TRANSACTION
LET row1 = SELECT * FROM ks.tbl WHERE k=0 AND c=0; <-- * selects
all columns
LET row2 = SELECT v FROM ks.tbl WHERE k=1 AND c=0;
SELECT row1, row2
IF row1 IS NULL AND row2.v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
I added the SELECT in the LET just so it's straightforward, you are
reading, and it's just like doing a regular select, but you are
assigning it to a variable.
I removed the confusing 'row1.v'and replaced it with 'row1'I can't
see why you would need the '.v'vs having the complete variable I
created in the statement above.
EOL
Patrick
On Thu, Aug 11, 2022 at 1:37 PM Caleb Rackliffe
<calebrackli...@gmail.com> wrote:
...and one more option...
5.) Introduce tuple assignments, removing all ambiguity around
row vs. column operations.
BEGIN TRANSACTION
LET row1 = * FROM ks.tbl WHERE k=0 AND c=0; <-- * selects all
columns
LET row2 = (v) FROM ks.tbl WHERE k=1 AND c=0;
SELECT row1.v, row2.v
IF row1 IS NULL AND row2.v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
On Thu, Aug 11, 2022 at 12:55 PM Caleb Rackliffe
<calebrackli...@gmail.com> wrote:
via Benedict, here is a 4th option:
4.) Similar to #2, but don't rely on the key element being
NULL.
If the read returns no result, x effectively becomes NULL.
Otherwise, it remains true/NOT NULL.
BEGIN TRANSACTION
LET x = true FROM ks.tbl WHERE k=0 AND c=0;
LET row2_v = v FROM ks.tbl WHERE k=1 AND c=0;
SELECT x, row2_v
IF x IS NULL AND row2_v = 3 THEN
INSERT INTO ks.tbl (k, c, v) VALUES (0, 0, 1);
END IF
COMMIT TRANSACTION
On Thu, Aug 11, 2022 at 12:12 PM Caleb Rackliffe
<calebrackli...@gmail.com> wrote:
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 inCASSANDRA-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
