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One of the major benefits of encoding data as JSON is that you do not have to predefine what structure or shape the data has. Further, the shape of the data can vary from one row to the next. The schema for JSON columns are defined by the data itself, rather than having to define the structure using SQL ahead of time. If a new field is needed, you simply create the appropriate data object in whatever programming language you are using, serialize it to JSON, and store it in VoltDB. This avoids the need to change the database schema anytime these temporary data structures change.
On the other hand, when using JSON columns you should be aware of the following drawbacks:
Your application must be intelligent enough to interpret the various structures that appear in the JSON columns.
Evaluating JSON data using the VoltDB JSON functions requires additional processing overhead. JSON is excellent for managing fluid data structures, but it is always significantly faster to used a predefined SQL schema when possible.
You must be sensitive to when indexes are needed, should query patterns or the data structures change. This may, for example, require you to add (or modify) indexes based on the existence of new JSON fields.
Another point of note is that there is a size limit for JSON values. In VoltDB, the VARCHAR columns used to store JSON values are limited to one megabyte (1048576 bytes). JSON support lets you augment the existing relational model within VoltDB. However, it is not intended or appropriate as a replacement for pure blob-oriented document stores.
V8.x Documentation