Chapter 60. Writing a Table Sampling Method

Chapter 60. Writing a Table Sampling Method
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60.1. Sampling Method Support Functions

+ 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. +

+ 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. +

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