Row level locking in CockroachDB - database

Question

I need to take row level lock for update and at the same time allow other select queries to get their intended row which is not locked.
What I observed was if I take a lock on row1, no other select queries are allowed which are searching for other rows.
I have below table schema -
CREATE TABLE lock_test(
id int NOT NULL DEFAULT unique_rowid(),
col_with_unique_index text NOT NULL,
other_col text NOT NULL,
CONSTRAINT "primary" PRIMARY KEY (id ASC),
UNIQUE INDEX unique_idx (col_with_unique_index ASC),
FAMILY "primary"(id, col_with_unique_index, other_col));
Inserted below 2 rows -
insert into lock_test(col_with_unique_index, other_col) values('val1', 'some_val');
insert into lock_test(col_with_unique_index, other_col) values('val2', 'some_val');
Opened 2 terminals -
1st terminal -
begin;
select * from lock_test where col_with_unique_index = 'val1' for update;
2nd terminal -
select * from lock_test where col_with_unique_index = 'val2';
Expected 2nd terminal to show the result for val2 but it did not(went into waiting), instead after I executed commit in 1st terminal 2nd terminal showed the result.
I tried changing my where clause from col_with_unique_index to id which is the primary key here, and this time 2nd terminal did not wait and displayed the expected result.
I'm unable to understand the behaviour here. Can I only take row level locks if I have primary key in my where clause?

Answer 1

I tried changing my where clause from col_with_unique_index to id which is the primary key here, and this time 2nd terminal did not wait and displayed the expected result.
I'm unable to understand the behaviour here. Can I only take row level locks if I have primary key in my where clause?
I suspect what happened here is that the number of rows in the table was very small and the optimizer determined that it would be cheaper to scan the primary index than to look at the unique index. We have tuned these hueristics a bit lately to account for this contention. You can force a scan against the secondary index using: select * from lock_test#unique_idx where col_with_unique_index = 'val2'; Or you can add more data and run stats (should run automatically).

Answer 2

I have tested row-level locking with cockroachDB v22.1.
In terminal 1:
SELECT version();
version
----------------------------------------------------------------------------------------
CockroachDB CCL v22.1.12 (x86_64-pc-linux-gnu, built 2022/12/12 19:53:40, go1.17.11)
(1 row)
CREATE TABLE my_table (
id INT PRIMARY KEY,
number INT
);
INSERT INTO my_table(id, number)
VALUES
(1, 100),
(2, 200),
(3, 300);
BEGIN TRANSACTION;
SELECT * FROM my_table WHERE id = 1 FOR UPDATE;
-- stop here
In terminal 2:
SELECT * FROM my_table; -- It will stop here
In terminal 1:
COMMIT TRANSACTION;
In terminal 2:
The output will continue.
id | number
-----+---------
1 | 100
2 | 200
3 | 300
(3 rows)