67.3. 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’ 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 65.2. All operator class members (procedures and operators) are mandatory.

Table 65.2. Procedure and Support Numbers for Minmax Operator Classes

To write an operator class for a complex data type which has values included within another type, it’s possible to use the inclusion support procedures alongside the corresponding operators, as shown in Table 65.3. It requires only a single additional function, which can be written in any language. More functions can be defined for additional functionality. All operators are optional. Some operators require other operators, as shown as dependencies on the table.

Table 65.3. Procedure and Support Numbers for Inclusion Operator Classes

Support procedure numbers 1-10 are reserved for the BRIN internal functions, so the SQL level functions start with number 11. Support function number 11 is the main function required to build the index. It should accept two arguments with the same data type as the operator class, and return the union of them. The inclusion operator class can store union values with different data types if it is defined with the STORAGE parameter. The return value of the union function should match the STORAGE data type.

Support procedure numbers 12 and 14 are provided to support irregularities of built-in data types. Procedure number 12 is used to support network addresses from different families which are not mergeable. Procedure number 14 is used to support empty ranges. Procedure number 13 is an optional but recommended one, which allows the new value to be checked before it is passed to the union function. As the BRIN framework can shortcut some operations when the union is not changed, using this function can improve index performance.

Both minmax and inclusion operator classes support cross-data-type operators, though with these the dependencies become more complicated. The minmax operator class requires a full set of operators to be defined with both arguments having the same data type. It allows additional data types to be supported by defining extra sets of operators. Inclusion operator class operator strategies are dependent on another operator strategy as shown in Table 65.3, or the same operator strategy as themselves. They require the dependency operator to be defined with the STORAGE data type as the left-hand-side argument and the other supported data type to be the right-hand-side argument of the supported operator. Seefloat4_minmax_ops as an example of minmax, and box_inclusion_ops as an example of inclusion.