pg_prewarm module provides a convenient way
to load relation data into either the operating system buffer cache
or the PostgreSQL buffer cache.
pg_prewarm(regclass, mode text default 'buffer', fork text default 'main', first_block int8 default null, last_block int8 default null) RETURNS int8
The first argument is the relation to be prewarmed. The second argument
is the prewarming method to be used, as further discussed below; the third
is the relation fork to be prewarmed, usually
The fourth argument is the first block number to prewarm
NULL is accepted as a synonym for zero). The fifth
argument is the last block number to prewarm (
means prewarm through the last block in the relation). The return value
is the number of blocks prewarmed.
There are three available prewarming methods.
issues asynchronous prefetch requests to the operating system, if this is
supported, or throws an error otherwise.
the requested range of blocks; unlike
prefetch, this is
synchronous and supported on all platforms and builds, but may be slower.
buffer reads the requested range of blocks into the
database buffer cache.
Note that with any of these methods, attempting to prewarm more blocks than
can be cached — by the OS when using
read, or by PostgreSQL when
buffer — will likely result in lower-numbered
blocks being evicted as higher numbered blocks are read in. Prewarmed data
also enjoys no special protection from cache evictions, so it is possible
that other system activity may evict the newly prewarmed blocks shortly
after they are read; conversely, prewarming may also evict other data from
cache. For these reasons, prewarming is typically most useful at startup,
when caches are largely empty.