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The 8kb is a limitation from block size. There is another limitation that one single account can only put 5 custom_jsons in one block (every 3 seconds).

Using Hive as a SQL database is not going to work. Designing around Hive is something that needs to be carefully thought out.

"kb" as KB (kilobyte) or Kb (kilobit)? Either way, a good amount of message can be stored in 8 kb. An SMS can be 140 bytes (0.14 Kilobyte) (1120 bits) (160 7-bit characters).