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The TSM handler function returns a palloc'dTsmRoutine 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.

voidSampleScanGetSampleSize (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 thebaserel->pages andbaserel->tuples numbers by that, being sure to round the results to integral values.) Theparamexprs list holds the expression(s) that are parameters to theTABLESAMPLE clause. It is recommended to useestimate_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). Thepages andtuples parameters are outputs.

voidInitSampleScan (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. TheSampleScanState node has already been created, but itstsm_state field is NULL. TheInitSampleScan function can palloc whatever internal state data is needed by the sampling method, and store a pointer to it innode->tsm_state. Information about the table to scan is accessible through other fields of theSampleScanState node (but note that thenode->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 forEXPLAIN andEndSampleScan.

This function can be omitted (set the pointer to NULL), in which caseBeginSampleScan must perform all initialization needed by the sampling method.

voidBeginSampleScan (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 theSampleScanState node (but note that thenode->ss.ss_currentScanDesc scan descriptor is not set up yet). Theparams array, of lengthnparams, contains the values of the parameters supplied in theTABLESAMPLE clause. These will have the number and types specified in the sampling method'sparameterTypes 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 theREPEATABLE value if one was given, or the result ofrandom() if not.

This function may adjust the fieldsnode->use_bulkread andnode->use_pagemode. Ifnode->use_bulkread istrue, 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 tofalse if the scan will visit only a small fraction of the table's pages. Ifnode->use_pagemode istrue, 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 tofalse 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 markedrepeatable_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 ofBeginSampleScan must lead to selecting the same tuples as before (if theTABLESAMPLE parameters and seed don't change).

BlockNumberNextSampleBlock (SampleScanState *node, BlockNumber nblocks);

Returns the block number of the next page to be scanned, orInvalidBlockNumber 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.

OffsetNumberNextSampleTuple (SampleScanState *node,                 BlockNumber blockno,                 OffsetNumber maxoffset);

Returns the offset number of the next tuple to be sampled on the specified page, orInvalidOffsetNumber if no tuples remain to be sampled.maxoffset is the largest offset number in use on the page.

voidEndSampleScan (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.