Improving relation extensions

For many years PostgreSQL has extended a table (or an index) block by block. In the case of a single writer process, this was usually fine. However, in cases of high-concurrency writing, writing a block at a time was a source of contention and suboptimal performance. From 9.6 onward, PostgreSQL started to extend tables by multiple blocks at a time. The number of blocks added at a time is 20 times the number of waiting processes.