sql/row: not deleting single-column family of composite column when updating with non-composite value

[michae2]

This is pretty niche, but this continue should sometimes be an if branch over MarshalLegacy if the reason we're skipping is because a value is non-composite:

cockroach/pkg/sql/row/writer.go

Line 142 in 78d326c

continue

Here's an example:

CREATE TABLE abc (a INT NOT NULL, b FLOAT NOT NULL, c INT, FAMILY (a), FAMILY (b), FAMILY (c));
-- note that -0::float is a composite value if b becomes part of the PK
INSERT INTO abc VALUES (4, -0, 6);
-- make b part of the PK
ALTER TABLE abc ADD PRIMARY KEY (a, b);
-- for example, this deletes the c column family KV as it should
UPDATE abc SET c = NULL WHERE a = 4 AND b = -0;
-- but this does not delete the b column family KV when b is no longer a composite value
UPDATE abc SET b = 0 WHERE a = 4 AND b = -0;
-- so now this still shows -0 (the composite value)
SELECT * FROM abc;

Looks like this was introduced in v22.2 from 5dff0e5

Jira issue: CRDB-42727