# 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.