That's absolutely a good question!
I have thought about this problem. Firstly, we should figure out how large the number of partitions is acceptable. From my experience, MySQL works well if we insert thousands of rows in a time, even tens of thousands are still acceptable. But if the order of magnitude grows to hundreds of thousands , even millions or more, MySQL would be very slow when inserting&retrieving those records.
When we are using HASH partition, since we can defined the buckets number of hash function, I think the number is under control. Normally it should be tens or hundreds . For RANGE and LIST partition, it works as well due to the partitions is enumerable. The worst situation I think is when we are using COLUMN partitions on a table, which is quite similar with hive's dynamic partition list below.
CREATE TABLE dst_tbl (key int, value string) PARTITIONED BY (col1 string, col2 it) AS
SELECT key, value, col1, col2 FROM src_tbl
Query users always have no knowledge about this table's value distribution. If the table is with high cardinality (a.k.a with so many distinct values), that should be a disaster for the below area
1. The number of files/directories on hdfs would be very large, big pressure for HDFS namenode's memory
2. As you mentioned, this would be a big problem for catalog.
Acutally, due to the above reasons. In Alibaba.com, my previous employer, which has one of the largest single hadoop cluster in the world, we disabled dynamic partitioning. I think you should run into the same problem when you are using column partitioning. I don't know why you guys decide to support such feature, could you give me some background about it? How can we benefit from column partitions?
It's good to know tajo will support indexes. I saw the binary search tree index in the branch. Actually, I am considering about adding lucene index into tajo, through which we can implements an online BA system on the top of tajo like senseidb. We can do group by aggregations on billions of rows with only a few milliseconds. If I implement it, we can put tajo into production in linkedin, my current employer.
Hyunsik Choi Jihoon Son
Thank you. Merry Christmas!