zlacker

>>hyzyla+(OP)
I've used PostgreSQL in the first two scenarios and would love an opportunity to in the third. It's worked really well and hasn't caused any problems under decent loads.

The one feature of Redis I'd love to have supported in PostgreSQL is to be able to set a TTL on a record. On a recent project where we could only save personal data for so long, it was a must have feature so we had to use Redis for that purpose instead.

>>petepe+V
Redis EXPIRE doesn't actually delete any data after it expires though. Active deletion happens at random, so you can easily still have expired values in memory months later:

> Redis keys are expired in two ways: a passive way, and an active way.

> A key is passively expired simply when some client tries to access it, and the key is found to be timed out.

> Of course this is not enough as there are expired keys that will never be accessed again. These keys should be expired anyway, so periodically Redis tests a few keys at random among keys with an expire set. All the keys that are already expired are deleted from the keyspace.

> Specifically this is what Redis does 10 times per second:

1. Test 20 random keys from the set of keys with an associated expire. 2. Delete all the keys found expired. 3. If more than 25% of keys were expired, start again from step 1.

So really it's not much better than doing `SELECT value from keys where key=? and expires > now()` with manual deletion. Though I agree that it can be more convenient.

>>phires+m2
hmmm, I disagree that it's not better. Select operation implies index scan most likely with O(log n), while GET operation is essentially O(2-3). And you also have to run DELETE on sql to remove the expired keys.

Oh, and i'm not entirely sure about the part about redis active expiry (disabled by default, default is remove expired on lookup - lazy); you're talking about key eviction which applies to all deleted keys and AFAIR happens only when certain watermarks are hit. Since it happens in ram, it's also faaaast, unlike SQL DELETE, which will definitely involve disk...