Can relational database scale horizontally

Question

After some googling I have found:

Note from mysql docs:

MySQL Cluster automatically shards (partitions) tables across nodes, enabling databases

Answer 1

It is possible but takes lots of maintenance efforts, Explanation -

Vertical Scaling of data (synonymous to Normalisation in SQL databases) is referred as splitting data column wise into multiple tables in order to reduce space redundancy. Example of user table -

Horizontal Scaling of data (synonymous to sharding) is referred as splitting row wise into multiple tables in order to reduce time taken to fetch data. Example of user table -

Key point to note here is as we can see tables in SQL databases are Normalised into multiple tables of related data. In order to shard data of such table on multiple machines, you would need to shard related normalised data accordingly which in turn would increase maintenance efforts. Like in the example presented above of SQL database,

Customer table which is related as one to many relation with Order table

If you move some rows of customer data onto other machine (referred as sharding) you would also need to move its related order data onto the same machine which would be troublesome task in case of multiple related tables.

Its convenient for NOSQL databases to shard out as they follow flat table structure (data is stored in aggregated form rather than normalised form).