PostgreSQL 9.5beta1 Documentation
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# 62.3. Extensibility

The BRIN interface has a high level of abstraction, requiring the access method implementer only to implement the semantics of the data type being accessed. The BRIN layer itself takes care of concurrency, logging and searching the index structure.

All it takes to get a BRIN access method working is to implement a few user-defined methods, which define the behavior of summary values stored in the index and the way they interact with scan keys. In short, BRIN combines extensibility with generality, code reuse, and a clean interface.

There are four methods that an operator class for BRIN must provide:

`BrinOpcInfo *opcInfo(Oid type_oid)`

  Returns internal information about the indexed columns' summary data. The return value must point to a palloc'd `BrinOpcInfo`, which has this definition:

      typedef struct BrinOpcInfo
      {
          /* Number of columns stored in an index column of this opclass */
          uint16      oi_nstored;

          /* Opaque pointer for the opclass' private use */
          void       *oi_opaque;

          /* Type cache entries of the stored columns */
          TypeCacheEntry *oi_typcache[FLEXIBLE_ARRAY_MEMBER];
      } BrinOpcInfo;

  `BrinOpcInfo`.`oi_opaque` can be used by the operator class routines to pass information between support procedures during an index scan.

`bool consistent(BrinDesc *bdesc, BrinValues *column, ScanKey key)`

  Returns whether the ScanKey is consistent with the given indexed values for a range. The attribute number to use is passed as part of the scan key.

`bool addValue(BrinDesc *bdesc, BrinValues *column, Datum newval, bool isnull)`

  Given an index tuple and an indexed value, modifies the indicated attribute of the tuple so that it additionally represents the new value. If any modification was done to the tuple, `true` is returned.

`bool unionTuples(BrinDesc *bdesc, BrinValues *a, BrinValues *b)`

  Consolidates two index tuples. Given two index tuples, modifies the indicated attribute of the first of them so that it represents both tuples. The second tuple is not modified.

The core distribution includes support for two types of operator classes: minmax and inclusion. Operator class definitions using them are shipped for in-core data types as appropriate. Additional operator classes can be defined by the user for other data types using equivalent definitions, without having to write any source code; appropriate catalog entries being declared is enough. Note that assumptions about the semantics of operator strategies are embedded in the support procedures's source code.

Operator classes that implement completely different semantics are also possible, provided implementations of the four main support procedures described above are written. Note that backwards compatibility across major releases is not guaranteed: for example, additional support procedures might be required in later releases.

To write an operator class for a data type that implements a totally ordered set, it is possible to use the minmax support procedures alongside the corresponding operators, as shown in Table 62-2. All operator class members (procedures and operators) are mandatory.

Table 62-2. Procedure and Support Numbers for Minmax Operator Classes

Operator class member | Object
--- | ---
Support Procedure 1 | function `brin_minmax_opcinfo()`
Support Procedure 2 | function `brin_minmax_add_value()`
Support Procedure 3 | function `brin_minmax_consistent()`
Support Procedure 4 | function `brin_minmax_union()`
Operator Strategy 1 | operator less-than
Operator Strategy 2 | operator less-than-or-equal-to
Operator Strategy 3 | operator equal-to
Operator Strategy 4 | operator greater-than-or-equal-to
Operator Strategy 5 | operator greater-than

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