What he means is something like non-reading-increments in hypertable.
How they function:
You write a+=1
if it doesn't exist in memory:
a=1
append +1 to commit-log
else:
a+=1 (in memory)
append to commit-log
After some time, 'a' is written to disk and the commit-log is checkpointed (so if a server crashes it doesn't have to read a very large commit log), and 'a' becomes immutable.
But you have to increment again the 'a' key, and it is immutable. So you create a new 'a':
And repeat again. After some time this is again persisted on disk and the commit log checkpointed.
Now you want to read the value of 'a':
If a merger has run, it reads different versions of data on disk and merges them, counters are merged and written as 1 key. So it reads 'a'.
If the merger has not run, it reads both versions of 'a', merges them in memory, and returns the value.
Now change '+1' to add_to_set(5). This is even better, because it updates the in-memory value, and if the hll doesn't change because '5' was already added to set, it doesn't even have to write/commit to log because no change is made.
How they function:
You write a+=1
After some time, 'a' is written to disk and the commit-log is checkpointed (so if a server crashes it doesn't have to read a very large commit log), and 'a' becomes immutable.But you have to increment again the 'a' key, and it is immutable. So you create a new 'a':
And repeat again. After some time this is again persisted on disk and the commit log checkpointed.
Now you want to read the value of 'a':
If a merger has run, it reads different versions of data on disk and merges them, counters are merged and written as 1 key. So it reads 'a'.
If the merger has not run, it reads both versions of 'a', merges them in memory, and returns the value.
Now change '+1' to add_to_set(5). This is even better, because it updates the in-memory value, and if the hll doesn't change because '5' was already added to set, it doesn't even have to write/commit to log because no change is made.