Online Alter Table


I saw the blog below. I have a question.

1) Does not require doubling the disk or memory use of the table while executing (creating a 2nd copy of the table without destroying the original table is not allowed)

MemSQL is the only distributed relational database able to achieve all three. For example, MySQL Cluster fails to do (1) – it copies the table in many cases.

2) Does not lock the table or prevent querying it for long periods of time (read or write) while running (under a second of blocking queries is OK)

Is this the only characteristic of MemSQL? What is the difference from other DBs (especially Oracle)? I want to know about the DBs with this characteristic.

I believe most of singlebox SQL Databases support online schema changes by our definition in that blog post - without copying data or locking (Oracle and SQL Server do). Fewer distributed SQL database have robust support for it. Almost all of them will either create duplicate copies of the data and “switch over” to the new copy when its ready or not have any online support at all (lock the table). I haven’t done a survey of how Vertica/Redshift have changed since that blog post was published but you can check the docs of various NewSQL databases and you can see they typically call out locking or copying of data.