64.3. 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 functions during an index scan.

bool consistent(BrinDesc *bdesc, BrinValues *column, ScanKey *keys, int nkeys)

Returns whether all the ScanKey entries are consistent with the given indexed values for a range. The attribute number to use is passed as part of the scan key. Multiple scan keys for the same attribute may be passed at once; the number of entries is determined by the nkeys parameter.

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. This is an older backward-compatible variant of the consistent function.

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.

An operator class for BRIN can optionally specify the following method:

void options(local_relopts *relopts)

Defines a set of user-visible parameters that control operator class behavior.

The options function is passed a pointer to a local_relopts struct, which needs to be filled with a set of operator class specific options. The options can be accessed from other support functions using the PG_HAS_OPCLASS_OPTIONS() and PG_GET_OPCLASS_OPTIONS() macros.

Since both key extraction of indexed values and representation of the key in BRIN are flexible, they may depend on user-specified parameters.

The core distribution includes support for four types of operator classes: minmax, minmax-multi, inclusion and bloom. 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 functions' source code.

Operator classes that implement completely different semantics are also possible, provided implementations of the four main support functions described above are written. Note that backwards compatibility across major releases is not guaranteed: for example, additional support functions 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 functions alongside the corresponding operators, as shown in Table 64.2. All operator class members (functions and operators) are mandatory.

Table 64.2. Function and Support Numbers for Minmax Operator Classes

Operator class memberObject
Support Function 1internal function brin_minmax_opcinfo()
Support Function 2internal function brin_minmax_add_value()
Support Function 3internal function brin_minmax_consistent()
Support Function 4internal function brin_minmax_union()
Operator Strategy 1operator less-than
Operator Strategy 2operator less-than-or-equal-to
Operator Strategy 3operator equal-to
Operator Strategy 4operator greater-than-or-equal-to
Operator Strategy 5operator greater-than

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 functions alongside the corresponding operators, as shown in Table 64.3. 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 64.3. Function and Support Numbers for Inclusion Operator Classes

Operator class memberObjectDependency
Support Function 1internal function brin_inclusion_opcinfo() 
Support Function 2internal function brin_inclusion_add_value() 
Support Function 3internal function brin_inclusion_consistent() 
Support Function 4internal function brin_inclusion_union() 
Support Function 11function to merge two elements 
Support Function 12optional function to check whether two elements are mergeable 
Support Function 13optional function to check if an element is contained within another 
Support Function 14optional function to check whether an element is empty 
Operator Strategy 1operator left-ofOperator Strategy 4
Operator Strategy 2operator does-not-extend-to-the-right-ofOperator Strategy 5
Operator Strategy 3operator overlaps 
Operator Strategy 4operator does-not-extend-to-the-left-ofOperator Strategy 1
Operator Strategy 5operator right-ofOperator Strategy 2
Operator Strategy 6, 18operator same-as-or-equal-toOperator Strategy 7
Operator Strategy 7, 16, 24, 25operator contains-or-equal-to 
Operator Strategy 8, 26, 27operator is-contained-by-or-equal-toOperator Strategy 3
Operator Strategy 9operator does-not-extend-aboveOperator Strategy 11
Operator Strategy 10operator is-belowOperator Strategy 12
Operator Strategy 11operator is-aboveOperator Strategy 9
Operator Strategy 12operator does-not-extend-belowOperator Strategy 10
Operator Strategy 20operator less-thanOperator Strategy 5
Operator Strategy 21operator less-than-or-equal-toOperator Strategy 5
Operator Strategy 22operator greater-thanOperator Strategy 1
Operator Strategy 23operator greater-than-or-equal-toOperator Strategy 1

Support function numbers 1 through 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 function numbers 12 and 14 are provided to support irregularities of built-in data types. Function number 12 is used to support network addresses from different families which are not mergeable. Function number 14 is used to support empty ranges. Function 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.

To write an operator class for a data type that implements only an equality operator and supports hashing, it is possible to use the bloom support procedures alongside the corresponding operators, as shown in Table 64.4. All operator class members (procedures and operators) are mandatory.

Table 64.4. Procedure and Support Numbers for Bloom Operator Classes

Operator class memberObject
Support Procedure 1internal function brin_bloom_opcinfo()
Support Procedure 2internal function brin_bloom_add_value()
Support Procedure 3internal function brin_bloom_consistent()
Support Procedure 4internal function brin_bloom_union()
Support Procedure 11function to compute hash of an element
Operator Strategy 1operator equal-to

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 one argument with the same data type as the operator class, and return a hash of the value.

The minmax-multi operator class is also intended for data types implementing a totally ordered set, and may be seen as a simple extension of the minmax operator class. While minmax operator class summarizes values from each block range into a single contiguous interval, minmax-multi allows summarization into multiple smaller intervals to improve handling of outlier values. It is possible to use the minmax-multi support procedures alongside the corresponding operators, as shown in Table 64.5. All operator class members (procedures and operators) are mandatory.

Table 64.5. Procedure and Support Numbers for minmax-multi Operator Classes

Operator class memberObject
Support Procedure 1internal function brin_minmax_multi_opcinfo()
Support Procedure 2internal function brin_minmax_multi_add_value()
Support Procedure 3internal function brin_minmax_multi_consistent()
Support Procedure 4internal function brin_minmax_multi_union()
Support Procedure 11function to compute distance between two values (length of a range)
Operator Strategy 1operator less-than
Operator Strategy 2operator less-than-or-equal-to
Operator Strategy 3operator equal-to
Operator Strategy 4operator greater-than-or-equal-to
Operator Strategy 5operator greater-than

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 64.3, 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. See float4_minmax_ops as an example of minmax, and box_inclusion_ops as an example of inclusion.