It is a common scenario in DynamoDB that you want to query all the items by a boolean value. e.g. you have a table that stores all the tasks, some running and some completed. And you want to periodically fetch out all the running tasks without scanning the whole table or separating them into two tables.
One way to do it is to use global secondary index. However, indexing in DynamoDB is different from that in a relational database. Just migrating the knowledge of MySQL indexing to DynamoDB might cause some confusions.
In DynamoDB, a GSI is very much like a table: It has its own partition key and sort key. It also splits its data into partitions. You can query a GSI but you must specify the partition key in the query operation, just like how you query a table. In fact, you can just think of a GSI as a table, except that each time you insert, update or delete an item, the same item gets inserted, updated or deleted in all the GSIs, which also means, the more GSIs you have, the slower it can be.
In the creation of a GSI, you can specify which fields you want to store in the GSI, which is called projection. The simplest is to only store the partition key and sort key of the item (which are required in order to refer back to the actual item), but you can also store all the fields at the cost of writing performance.
The primary key of a GSI has two advantages over that of the table, which makes it possible to query by a boolean value.
- If the partition key or sort key of the GSI is missing in the item, the item won’t be inserted to the GSI. Or if it’s already in the GSI, then it will be deleted from the GSI.
- The composition of the partition key and sort key doesn’t need to be unique in the GSI.
Making use of this feature, we can support querying by task status by creating a GSI with this composite primary key:
- Use the task ID (or any other field whose values are distributed in a large value range) as the partition key;
- Use a boolean
runningfield as the sort key: the presence of the field means running and the absence means completed. Or you can store more informative values in the sort key, like
runningCount, which indicates how many times the task has been executed, or
runningSince, which indicates when the task started.