MySql : Does it worth to convert a column with type DATETIME to DATE?

Question

At the start of a project, a DATETIME column made sense. But now, 5 years after, requierments changed the time part have never be used.

My table has about 100k lines and grows with about 500 lines per month.

Does it worth to convert the column with a type DATETIME to DATE?

• I mean, does a DATETIME field takes more size compare to DATE field?
• Can I expect some disk space saving after column conversion?
• Is it more faster for the DB to search in a DATE field?

I am using MySQL 5.7

Answer 1

I mean, does a DATETIME field takes more size compare to DATE field?

That's true. DATE needs 3 bytes, DATETIME needs 5-8 bytes.

Is it more faster for the DB to search in a DATE field?

In most cases the difference is below the measuring accuracy.

Can I expect some disk space saving after column conversion?

I doubt. Clustering effect. The larger the row_size, the less likely it is that the table file size will change.

Approximate explaination (not accurate, idea only).

The data is stored in the table file not as stream, but in blocks. There is some percent range of data fill in a block - when the real percent comes out of it, the blocks splits or joins.

Imagine that

• block size is 16 kbytes = 16384 bytes
• record size is 100 bytes
• percent range is 40-80%, average 60%

In general the block stores (16384 * 60%) / 100 = 98 records in average.

We change DATETIME without fractional part (5 bytes) to DATE (3 bytes). New record size is 100 - 5 + 3 = 98 bytes.

So the block now stores (16384 * 60%) / 98 = 100 records in average.

The amount of blocks decreases by 2% only. Taking into account that the percent range varies twice, the file size decrease probability is 2% higher than increase probability. I.e. in most cases yuou cannot predict the change in real storage size.

Answer 2

• yes at least the double see https://dev.mysql.com/doc/refman/8.0/en/storage-requirements.html#data-types-storage-reqs-date-time
• with that small number of rows 50-60 kb
• when then very small performance differences, not that i noticed , but you have a small database, that is growing slowly, and hardware changes and speeds up every two years.

You could think about mysql 8 , it is faster than the 5.x versions and has a lot of new function, specially the window functions and Regular expression