MySQL index based on last digit of INT column - mysql

Question

Is it possible to create an index in MySQL for the last digit of an int column?
Based on this answer i have created partitions based on last digit of an int column
CREATE TABLE partition_test(
textfiled INT,
cltext TEXT,
reindexedAt TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
indexedAt TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
status TINYINT(2),
postId INT)
PARTITION BY HASH(MOD(postId, 10))
PARTITIONS 10;
I'm trying to create an index for the last digit of postId for optimizing queries time. Is there any way to do this or a simple index on postId is enough?
Some failed tries:
CREATE INDEX postLastDigit USING HASH ON partition_test (MOD(postId, 10));
(1064, u"You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'MOD(postId, 10))' at line 1")
and
CREATE INDEX postLastDigit ON partition_test (MOD(postId, 10));
(1064, u"You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'MOD(postId, 10))' at line 1")
UPDATE:
The table has more than 100M row.
My goal is to optimize queries like:
1)
SELECT cltext FROM partition_tables
WHERE postId in (<INT>, <INT>)
AND status IS NOT NULL
2)
SELECT cltext FROM partition_tables
WHERE postId in (<INT>, <INT>)
AND status IS NOT NULL
AND reindexedAt BETWEEN (<DATE>, <DATE>)
MariaDB version: 10.1.23-MariaDB-9+deb9u1

Answer 1

What query are you trying to speed up? Without any indexes on the table, any query will have to scan the entire table! If you want speed, first look to indexing.
If your query is SELECT ... WHERE post_id = 123, your Partitioning might make it run about 10 times as fast. But INDEX(post_id), with or without partitioning, will make it run hundreds of times as fast.
Please provide the SELECTs so we can help you speed them up.
(OK, if you are just playing around with partitioning, the others have given you viable answers.)
"Partition Pruning" is rarely faster than a suitable index that starts with the pruning column.
After you solve your stated hashing problem, please report back whether the queries any faster than using an index. Even pitted against an index, I predict partitioning will not run faster, and may even run a little slower.

Answer 2

You have tagged your question with mariadb and mysql. If you are using a resonably recent version of MariaDB, you can use generated columns for indexing. If you are using MySQL, you can do the same if your MySQL version is at least 5.7.
If you are using a lower version of MySQL, you could create an additional column in your table where you store the last digit of postId for each row, and use that column for indexing / partitioning.
This would mean minimal changes to your application code: Before inserting or updating, get the last digit of postId first, and insert / update one more field. As an alternative, you eventually could use triggers to automatically fill that additional column.

Answer 3

Use virtual columns. In MariaDB 10.2, you can create index on virtual aka generated column, like this
CREATE TABLE t (
num int,
last_digit int(1) AS (num % 10) VIRTUAL,
KEY index_last_digit (last_digit)
)
Then you can use last_digit in your queries, i.e SELECT ... WHERE last_digit=1
In older versions of MariaDB, 5.2 to 10.1 , you'd need to specify PERSISTENT attribute rather than VIRTUAL, because non-persistent generated columns could not be indexed.