Different values written at different times
A database generally allows any value to be updated at any time. This means that within a single database you may have some values that were written five milliseconds ago, and some values that were written five years ago.
When you deploy a new version of your application (of a server-side application, at least), you may entirely replace the old version with the new version within a few minutes. The same is not true of database contents: the five-year-old data will still be there, in the original encoding, unless you have explicitly rewritten it since then. This observation is sometimes summed up as data outlives code.
Rewriting (migrating) data into a new schema is certainly possible, but it’s an expensive thing to do on a large dataset, so most databases avoid it if possible. Most relational databases allow simple schema changes, such as adding a new column with a null default value, without rewriting existing data. When an old row is read, the database fills in nulls for any columns that are missing from the encoded data on disk. LinkedIn’s document database Espresso uses Avro for storage, allowing it to use Avro’s schema evolution rules .
Schema evolution thus allows the entire database to appear as if it was encoded with a single schema, even though the underlying storage may contain records encoded with various historical versions of the schema.
-  Except for MySQL, which often rewrites an entire table even though it is not strictly necessary, as mentioned in “Schema flexibility in the document model” on page 39.