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.
- 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.