Table of Contents
  PostgreSQL's implementation of the TABLESAMPLE
  clause supports custom table sampling methods, in addition to
  the BERNOULLI and SYSTEM methods that are required
  by the SQL standard.  The sampling method determines which rows of the
  table will be selected when the TABLESAMPLE clause is used.
 
At the SQL level, a table sampling method is represented by a single SQL function, typically implemented in C, having the signature
method_name(internal) RETURNS tsm_handler
  The name of the function is the same method name appearing in the
  TABLESAMPLE clause.  The internal argument is a dummy
  (always having value zero) that simply serves to prevent this function from
  being called directly from an SQL command.
  The result of the function must be a palloc'd struct of
  type TsmRoutine, which contains pointers to support functions for
  the sampling method.  These support functions are plain C functions and
  are not visible or callable at the SQL level.  The support functions are
  described in Section 58.1.
 
  In addition to function pointers, the TsmRoutine struct must
  provide these additional fields:
 
List *parameterTypes
     This is an OID list containing the data type OIDs of the parameter(s)
     that will be accepted by the TABLESAMPLE clause when this
     sampling method is used.  For example, for the built-in methods, this
     list contains a single item with value FLOAT4OID, which
     represents the sampling percentage.  Custom sampling methods can have
     more or different parameters.
    
bool repeatable_across_queries
     If true, the sampling method can deliver identical samples
     across successive queries, if the same parameters
     and REPEATABLE seed value are supplied each time and the
     table contents have not changed.  When this is false,
     the REPEATABLE clause is not accepted for use with the
     sampling method.
    
bool repeatable_across_scans
     If true, the sampling method can deliver identical samples
     across successive scans in the same query (assuming unchanging
     parameters, seed value, and snapshot).
     When this is false, the planner will not select plans that
     would require scanning the sampled table more than once, since that
     might result in inconsistent query output.
    
  The TsmRoutine struct type is declared
  in src/include/access/tsmapi.h, which see for additional
  details.
 
  The table sampling methods included in the standard distribution are good
  references when trying to write your own.  Look into
  the src/backend/access/tablesample subdirectory of the source
  tree for the built-in sampling methods, and into the contrib
  subdirectory for add-on methods.