Why Uber Engineering Switched from Postgres to MySQL
When we updated the birth year for al-Khwārizmī, we didn’t actually change his primary key, nor did we change his first and last name. However, these indexes still must be updated with the creation of a new row tuple in the database for the row record. For tables with a large number of secondary indexes, these superfluous steps can cause enormous inefficiencies. For instance, if we have a table with a dozen indexes defined on it, an update to a field that is only covered by a single index must be propagated into all 12 indexes to reflect the ctid for the new row.
.. This write amplification issue naturally translates into the replication layer as well
.. In cases where Postgres replication happens purely within a single data center, the replication bandwidth may not be a problem.
.. However, the verbosity of the Postgres replication protocol can still cause an overwhelming amount of data for a database that uses a lot of indexes.