Can Cassandra row cache only cache on specific columns? - cassandra

Question

I have a C* table which are wide and some columns are read-heavy. I am considering using row cache but do not know if row cache can store specific columns. If all the cells of that row need to be stored, the content to be cached may grow too fast and defeat the purpose.
The schema is like follows:
CREATE TABLE tb1 (
pk1 int,
ck1 int,
read_heavy_col1 int,
read_heavy_col2 int,
normal_col1 int,
normal_col2 int,
...
PRIMARY KEY (pk1, ck1)
)
The question is if row cache is able to cache pk1, ck1, read_heavy_col1, read_heavy_col2 only and ignores normal_col1, normal_col2, ....
According to DataStax Configuring data caches,
If the newly cached data does not include all cells configured by
user, Cassandra performs another read.
Does that mean C* can cache only the columns of interest?

Answer 1

You can instead of using a large number of columns, instead have a single clustering key that the heavy read columns sort earlier as, then limit the rows_per_partition to just capture those.
You will probably not get a whole lot of benefit vs just caching entire row. Also while the key cache is generally huge for read performance the row cache really only helps in very specific scenarios (and hurts in some) so be sure to benchmark it as the os cache is usually sufficient to keep the important bits in memory.

Answer 2

Answering my own questions:
Cassandra will cache all columns of that row.
From "Cassandra The Definitive Guide" (2nd edition) by Jeff Carpenter & Eben Hewitt:
The row cache caches entire rows and can speed up read access for frequently accessed rows, at the cost of more memory usage.
Experiment:
abc#cqlsh:test> SELECT pk1, ck1, read_heavy_col1 FROM tb1 WHERE pk1=2 and ck1=1;
pk1 | ck1 | read_heavy_col1
-----+-----+-----------------
2 | 1 | 1
(1 rows)
Tracing session: 53aac630-11be-11e9-9cb3-fbffb1c1e13b
activity | timestamp | source | source_elapsed
--------------------------------------------------------------------------------------------------------+----------------------------+--------------+----------------
Execute CQL3 query | 2019-01-06 22:21:15.667000 | x.x.x.x | 0
Parsing SELECT pk1, ck1, read_heavy_col1 FROM tb1 WHERE pk1=2 and ck1=1; [Native-Transport-Requests-1] | 2019-01-06 22:21:15.668000 | x.x.x.x | 310
Preparing statement [Native-Transport-Requests-1] | 2019-01-06 22:21:15.668000 | x.x.x.x | 568
Executing single-partition query on roles [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1259
Acquiring sstable references [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1345
Key cache hit for sstable 2 [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1475
Skipped 0/1 non-slice-intersecting sstables, included 0 due to tombstones [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1569
Merged data from memtables and 1 sstables [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1768
Read 1 live rows and 0 tombstone cells [ReadStage-2] | 2019-01-06 22:21:15.669000 | x.x.x.x | 1869
Executing single-partition query on roles [ReadStage-3] | 2019-01-06 22:21:15.672000 | x.x.x.x | 4579
Acquiring sstable references [ReadStage-3] | 2019-01-06 22:21:15.672000 | x.x.x.x | 4701
Key cache hit for sstable 2 [ReadStage-3] | 2019-01-06 22:21:15.672000 | x.x.x.x | 4812
Skipped 0/1 non-slice-intersecting sstables, included 0 due to tombstones [ReadStage-3] | 2019-01-06 22:21:15.672000 | x.x.x.x | 4891
Merged data from memtables and 1 sstables [ReadStage-3] | 2019-01-06 22:21:15.672000 | x.x.x.x | 5001
Read 1 live rows and 0 tombstone cells [ReadStage-3] | 2019-01-06 22:21:15.673000 | x.x.x.x | 5081
Row cache miss [ReadStage-3] | 2019-01-06 22:21:15.674000 | x.x.x.x | 6227
Executing single-partition query on tb1 [ReadStage-3] | 2019-01-06 22:21:15.674000 | x.x.x.x | 6387
Acquiring sstable references [ReadStage-3] | 2019-01-06 22:21:15.674000 | x.x.x.x | 6445
Skipped 0/0 non-slice-intersecting sstables, included 0 due to tombstones [ReadStage-3] | 2019-01-06 22:21:15.674001 | x.x.x.x | 6514
Caching 3 rows [ReadStage-3] | 2019-01-06 22:21:15.674001 | x.x.x.x | 6647
Merged data from memtables and 0 sstables [ReadStage-3] | 2019-01-06 22:21:15.675000 | x.x.x.x | 7231
Read 1 live rows and 0 tombstone cells [ReadStage-3] | 2019-01-06 22:21:15.675000 | x.x.x.x | 7443
Request complete | 2019-01-06 22:21:15.676482 | x.x.x.x | 9482
Then the rows are cached, fetching a normal column:
abc#cqlsh:test> SELECT pk1, ck1, normal_col2 FROM tb1 WHERE pk1=2 and ck1=1;
pk1 | ck1 | normal_col2
-----+-----+-------------
2 | 1 | 1
(1 rows)
Tracing session: a178ae90-11be-11e9-9cb3-fbffb1c1e13b
activity | timestamp | source | source_elapsed
----------------------------------------------------------------------------------------------------+----------------------------+--------------+----------------
Execute CQL3 query | 2019-01-06 22:23:26.201000 | x.x.x.x | 0
Parsing SELECT pk1, ck1, normal_col2 FROM tb1 WHERE pk1=2 and ck1=1; [Native-Transport-Requests-1] | 2019-01-06 22:23:26.202000 | x.x.x.x | 205
Preparing statement [Native-Transport-Requests-1] | 2019-01-06 22:23:26.202000 | x.x.x.x | 393
Executing single-partition query on roles [ReadStage-3] | 2019-01-06 22:23:26.202000 | x.x.x.x | 968
Acquiring sstable references [ReadStage-3] | 2019-01-06 22:23:26.203000 | x.x.x.x | 1413
Key cache hit for sstable 2 [ReadStage-3] | 2019-01-06 22:23:26.203000 | x.x.x.x | 1564
Skipped 0/1 non-slice-intersecting sstables, included 0 due to tombstones [ReadStage-3] | 2019-01-06 22:23:26.203000 | x.x.x.x | 1685
Merged data from memtables and 1 sstables [ReadStage-3] | 2019-01-06 22:23:26.203000 | x.x.x.x | 1841
Read 1 live rows and 0 tombstone cells [ReadStage-3] | 2019-01-06 22:23:26.203000 | x.x.x.x | 1930
Executing single-partition query on roles [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2307
Acquiring sstable references [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2375
Key cache hit for sstable 2 [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2475
Skipped 0/1 non-slice-intersecting sstables, included 0 due to tombstones [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2584
Merged data from memtables and 1 sstables [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2691
Read 1 live rows and 0 tombstone cells [ReadStage-5] | 2019-01-06 22:23:26.204000 | x.x.x.x | 2761
Row cache hit [ReadStage-3] | 2019-01-06 22:23:26.205000 | x.x.x.x | 3301
Read 1 live rows and 0 tombstone cells [ReadStage-3] | 2019-01-06 22:23:26.205000 | x.x.x.x | 3489
Request complete | 2019-01-06 22:23:26.204726 | x.x.x.x | 3726
Clearly C* will cache the entire row rather than specific columns.