The 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 main.
   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 (NULL
   means prewarm through the last block in the relation).  The return value
   is the number of blocks prewarmed.
  
   There are three available prewarming methods.  prefetch
   issues asynchronous prefetch requests to the operating system, if this is
   supported, or throws an error otherwise.  read reads
   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 prefetch or
   read, or by PostgreSQL when
   using 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.
  
   Robert Haas <rhaas@postgresql.org>