Chapter 58. Writing a Table Sampling Method
Table of Contents
PostgreSQL's implementation of theTABLESAMPLE clause supports custom table sampling methods, in addition to theBERNOULLI andSYSTEM methods that are required by the SQL standard. The sampling method determines which rows of the table will be selected when theTABLESAMPLE 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 theTABLESAMPLE clause. Theinternal 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 typeTsmRoutine, 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 inSection 58.1.
In addition to function pointers, theTsmRoutine struct must provide these additional fields:
List *parameterTypesThis is an OID list containing the data type OIDs of the parameter(s) that will be accepted by the
TABLESAMPLEclause when this sampling method is used. For example, for the built-in methods, this list contains a single item with valueFLOAT4OID, which represents the sampling percentage. Custom sampling methods can have more or different parameters.bool repeatable_across_queriesIf
true, the sampling method can deliver identical samples across successive queries, if the same parameters andREPEATABLEseed value are supplied each time and the table contents have not changed. When this isfalse, theREPEATABLEclause is not accepted for use with the sampling method.bool repeatable_across_scansIf
true, the sampling method can deliver identical samples across successive scans in the same query (assuming unchanging parameters, seed value, and snapshot). When this isfalse, the planner will not select plans that would require scanning the sampled table more than once, since that might result in inconsistent query output.
TheTsmRoutine struct type is declared insrc/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 thesrc/backend/access/tablesample subdirectory of the source tree for the built-in sampling methods, and into thecontrib subdirectory for add-on methods.