Why require the PRIMARY KEY column to be present in a PARTITIONED BY clause?


#1

can you help me understand what I am reading in this documentation?https://crate.io/docs/reference/sql/partitioned_tables.html

In these example tables, column id long is not a primary_key ; indeed, id could not be a primary key here, because as noted below “if a primary key is set, it must be present in the PARTITION BY clause”

In my app, I’ve historically had a primary key on id string NOT NULL , but now I want to add partitioning on this table, on a generated date column just like in the example partition_date timestamp GENERATED ALWAYS AS date_trunc('day', created_at) . I’ve read that partitioning on a date column will help with speed of queries scoped by time period (counting all today’s records, for example, would only hit today’s partitions), and helping me archive older frames of data (e.g. anything > 180 days), but I don’t want to lose performance of single PK lookups.

So since I can’t do just PARTITIONED BY (partition_date) , is it best if I…

a) Remove the primary key constraint from id ? I’m nervous this would affect my performance for single row lookups! In this context, it makes sense that PK must be in the partition key, because to lookup WHERE id = "abc-123" should ideally only have to hit a single node.

or

b) use both columns as partition key, like PARTITIONED BY (id, partition_date) – This seems weird, because instinctively, I want to assume that id would have high cardinality and be a bad choice for a partition column, and ‘day’ or ‘month’ would be better, like is shown in the example on your docs. In this case, is my PK lookup hitting every partition, or does it know exactly where to go? If I run an aggregate query scoped to today only, will it hit every partition or only the one holding today’s data?


#2

So that’s a great question! Since partitions are “sub tables” of sort, that help reducing the size of the queried data.

A primary key influences routing in CrateDB, so adding it to a partitioned table (which requires more extensive routing) will deny any non-primary key column in the partitioned by clause. Thus, your options are as follows:

  • a) While this removes the ability to do PK lookups effectively, this seems like a sensible option to do - regular string lookups can be accelerated by using a fulltext index - but it is also going to remove read-after-write-consistent primary key lookups add. Depending on the way you generate the primary key, it might be feasible to use the internal _id column instead (for lookups), or issue a REFRESH TABLE before a id-lookup.
  • b) Will result in as many partitions as there are primary keys (and since they are unique …) - so this option will create too many partitions

Since option b) will result in chaos, I’d recommend option a). However, if the primary key lookups are crucial to your application and the expected amount of data is not that huge (a couple of million are fine - depending on cluster size and machine specs of course), it might work just fine without partitioning!