The TSM handler function returns a palloc'd TsmRoutine struct
   containing pointers to the support functions described below.  Most of
   the functions are required, but some are optional, and those pointers can
   be NULL.
  
void
SampleScanGetSampleSize (PlannerInfo *root,
                         RelOptInfo *baserel,
                         List *paramexprs,
                         BlockNumber *pages,
                         double *tuples);
   This function is called during planning.  It must estimate the number of
   relation pages that will be read during a sample scan, and the number of
   tuples that will be selected by the scan.  (For example, these might be
   determined by estimating the sampling fraction, and then multiplying
   the baserel->pages and baserel->tuples
   numbers by that, being sure to round the results to integral values.)
   The paramexprs list holds the expression(s) that are
   parameters to the TABLESAMPLE clause.  It is recommended to
   use estimate_expression_value() to try to reduce these
   expressions to constants, if their values are needed for estimation
   purposes; but the function must provide size estimates even if they cannot
   be reduced, and it should not fail even if the values appear invalid
   (remember that they're only estimates of what the run-time values will be).
   The pages and tuples parameters are outputs.
  
void
InitSampleScan (SampleScanState *node,
                int eflags);
   Initialize for execution of a SampleScan plan node.
   This is called during executor startup.
   It should perform any initialization needed before processing can start.
   The SampleScanState node has already been created, but
   its tsm_state field is NULL.
   The InitSampleScan function can palloc whatever internal
   state data is needed by the sampling method, and store a pointer to
   it in node->tsm_state.
   Information about the table to scan is accessible through other fields
   of the SampleScanState node (but note that the
   node->ss.ss_currentScanDesc scan descriptor is not set
   up yet).
   eflags contains flag bits describing the executor's
   operating mode for this plan node.
  
   When (eflags & EXEC_FLAG_EXPLAIN_ONLY) is true,
   the scan will not actually be performed, so this function should only do
   the minimum required to make the node state valid for EXPLAIN
   and EndSampleScan.
  
   This function can be omitted (set the pointer to NULL), in which case
   BeginSampleScan must perform all initialization needed
   by the sampling method.
  
void
BeginSampleScan (SampleScanState *node,
                 Datum *params,
                 int nparams,
                 uint32 seed);
   Begin execution of a sampling scan.
   This is called just before the first attempt to fetch a tuple, and
   may be called again if the scan needs to be restarted.
   Information about the table to scan is accessible through fields
   of the SampleScanState node (but note that the
   node->ss.ss_currentScanDesc scan descriptor is not set
   up yet).
   The params array, of length nparams, contains the
   values of the parameters supplied in the TABLESAMPLE clause.
   These will have the number and types specified in the sampling
   method's parameterTypes list, and have been checked
   to not be null.
   seed contains a seed to use for any random numbers generated
   within the sampling method; it is either a hash derived from the
   REPEATABLE value if one was given, or the result
   of random() if not.
  
   This function may adjust the fields node->use_bulkread
   and node->use_pagemode.
   If node->use_bulkread is true, which it is by
   default, the scan will use a buffer access strategy that encourages
   recycling buffers after use.  It might be reasonable to set this
   to false if the scan will visit only a small fraction of the
   table's pages.
   If node->use_pagemode is true, which it is by
   default, the scan will perform visibility checking in a single pass for
   all tuples on each visited page.  It might be reasonable to set this
   to false if the scan will select only a small fraction of the
   tuples on each visited page.  That will result in fewer tuple visibility
   checks being performed, though each one will be more expensive because it
   will require more locking.
  
   If the sampling method is
   marked repeatable_across_scans, it must be able to
   select the same set of tuples during a rescan as it did originally, that is
   a fresh call of BeginSampleScan must lead to selecting the
   same tuples as before (if the TABLESAMPLE parameters
   and seed don't change).
  
BlockNumber NextSampleBlock (SampleScanState *node);
   Returns the block number of the next page to be scanned, or
   InvalidBlockNumber if no pages remain to be scanned.
  
This function can be omitted (set the pointer to NULL), in which case the core code will perform a sequential scan of the entire relation. Such a scan can use synchronized scanning, so that the sampling method cannot assume that the relation pages are visited in the same order on each scan.
OffsetNumber
NextSampleTuple (SampleScanState *node,
                 BlockNumber blockno,
                 OffsetNumber maxoffset);
   Returns the offset number of the next tuple to be sampled on the
   specified page, or InvalidOffsetNumber if no tuples remain to
   be sampled.  maxoffset is the largest offset number in use
   on the page.
  
    NextSampleTuple is not explicitly told which of the offset
    numbers in the range 1 .. maxoffset actually contain valid
    tuples.  This is not normally a problem since the core code ignores
    requests to sample missing or invisible tuples; that should not result in
    any bias in the sample.  However, if necessary, the function can
    examine node->ss.ss_currentScanDesc->rs_vistuples[]
    to identify which tuples are valid and visible.  (This
    requires node->use_pagemode to be true.)
   
    NextSampleTuple must not assume
    that blockno is the same page number returned by the most
    recent NextSampleBlock call.  It was returned by some
    previous NextSampleBlock call, but the core code is allowed
    to call NextSampleBlock in advance of actually scanning
    pages, so as to support prefetching.  It is OK to assume that once
    sampling of a given page begins, successive NextSampleTuple
    calls all refer to the same page until InvalidOffsetNumber is
    returned.
   
void EndSampleScan (SampleScanState *node);
End the scan and release resources. It is normally not important to release palloc'd memory, but any externally-visible resources should be cleaned up. This function can be omitted (set the pointer to NULL) in the common case where no such resources exist.