Chopping off SHA-256 entropy? I have a NoSQL key-value database. I want to insert couple million of records to it.

For the key generation, I'm, using a prefix (something like data-one-) and then concat the SHA-256 hash of something unique to the value stored. So the end key might look like 

data-one-0000fdb60e164cf0bf07cef647354d26ed17c9492ca4bbf4114878325871fa1d


The problem is that the key is somewhat large and it will have impact of the DB performance. I want to chop off some of the hash to get a 64-bit string (so, 8 chars). So I'd chop the above key to 

data-one-0000fdb6


But will it leave enough entropy for the remaining key? The DB will get large, and I don't want collisions, obviously. Will another hash be more suitable for this use, or is trimming SHA-256 fine?
Assuming the hash function is good, and SHA-256 is widely believed to be so, the probability of no collisions in $k$ samples into a range of size $n$ (e.g., obtained by selecting a subset of characters) is upper bounded by $exp(-k^2/2n).$

For you, say $k=2^{21}=2,097,152$ (couple million) and $n=2^{64}$ means that your probability of no collision is roughly

$$exp(-2^{42}/2^{65})=exp(-2^{-21})\approx 1-1.2\times 10^{-7}.$$

If you had 8 times more inputs (16 million plus) this would become

$$ exp(-2^{48}/2^{65})=exp(-2^{-17})\approx 1-7.6\times 10^{-6}$$

so probability of collision would be more than one in a million.