# 60. Writing a Table Sampling Method 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 60.1](https://www.postgresql.org/docs/15/tablesample-support-functions.html). 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.postgresql.tw/15/internals/60.-writing-a-table-sampling-method.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.