62.3. Extensibility
Traditionally, implementing a new index access method meant a lot of difficult work. It was necessary to understand the inner workings of the database, such as the lock manager and Write-Ahead Log. TheGiST interface has a high level of abstraction, requiring the access method implementer only to implement the semantics of the data type being accessed. TheGiST layer itself takes care of concurrency, logging and searching the tree structure. This extensibility should not be confused with the extensibility of the other standard search trees in terms of the data they can handle. For example,PostgreSQL supports extensible B-trees and hash indexes. That means that you can usePostgreSQL to build a B-tree or hash over any data type you want. But B-trees only support range predicates ( So if you index, say, an image collection with aPostgreSQL B-tree, you can only issue queries such as“is imagex equal to imagey”,“is imagex less than imagey” and“is imagex greater than imagey”. Depending on how you define“equals”,“less than” and“greater than” in this context, this could be useful. However, by using aGiST based index, you could create ways to ask domain-specific questions, perhaps“find all images of horses” or“find all over-exposed images”. All it takes to get aGiST access method up and running is to implement several user-defined methods, which define the behavior of keys in the tree. Of course these methods have to be pretty fancy to support fancy queries, but for all the standard queries (B-trees, R-trees, etc.) they're relatively straightforward. In short,GiST combines extensibility along with generality, code reuse, and a clean interface. There are seven methods that an index operator class forGiST must provide, and two that are optional. Correctness of the index is ensured by proper implementation of the Given an index entry TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: Here, Depending on which operators you have included in the class, the data type of This method consolidates information in the tree. Given a set of entries, this function generates a new index entry that represents all the given entries. TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: As you can see, in this skeleton we're dealing with a data type where The result of the As shown above, the Converts the data item into a format suitable for physical storage in an index page. TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: You have to adapt The reverse of the TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: The above skeleton is suitable for the case where no decompression is needed. Returns a value indicating the“cost” of inserting the new entry into a particular branch of the tree. Items will be inserted down the path of least TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: For historical reasons, the The When an index page split is necessary, this function decides which entries on the page are to stay on the old page, and which are to move to the new page. TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: Notice that the Like Returns true if two index entries are identical, false otherwise. (An“index entry” is a value of the index's storage type, not necessarily the original indexed column's type.) TheSQL declaration of the function must look like this: And the matching code in the C module could then follow this skeleton: For historical reasons, the Given an index entry All the GiST support methods are normally called in short-lived memory contexts; that is,<,=,>), and hash indexes only support equality queries.same,consistent andunion methods, while efficiency (size and speed) of the index will depend on thepenalty andpicksplit methods. The remaining two basic methods arecompress anddecompress, which allow an index to have internal tree data of a different type than the data it indexes. The leaves are to be of the indexed data type, while the other tree nodes can be of any C struct (but you still have to followPostgreSQL data type rules here, see aboutvarlena for variable sized data). If the tree's internal data type exists at the SQL level, theSTORAGE option of theCREATE OPERATOR CLASS command can be used. The optional eighth method isdistance, which is needed if the operator class wishes to support ordered scans (nearest-neighbor searches). The optional ninth methodfetch is needed if the operator class wishes to support index-only scans.consistentp and a query valueq, this function determines whether the index entry is“consistent” with the query; that is, could the predicate“indexed_columnindexable_operatorq” be true for any row represented by the index entry? For a leaf index entry this is equivalent to testing the indexable condition, while for an internal tree node this determines whether it is necessary to scan the subtree of the index represented by the tree node. When the result istrue, arecheck flag must also be returned. This indicates whether the predicate is certainly true or only possibly true. Ifrecheck =false then the index has tested the predicate condition exactly, whereas ifrecheck =true the row is only a candidate match. In that case the system will automatically evaluate theindexable_operator against the actual row value to see if it is really a match. This convention allowsGiST to support both lossless and lossy index structures.CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)RETURNS boolAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_consistent);Datummy_consistent(PG_FUNCTION_ARGS){ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); data_type *query = PG_GETARG_DATA_TYPE_P(1); StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2); /* Oid subtype = PG_GETARG_OID(3); */ bool *recheck = (bool *) PG_GETARG_POINTER(4); data_type *key = DatumGetDataType(entry->key); bool retval; /* * determine return value as a function of strategy, key and query. * * Use GIST_LEAF(entry) to know where you're called in the index tree, * which comes handy when supporting the = operator for example (you could * check for non empty union() in non-leaf nodes and equality in leaf * nodes). */ *recheck = true; /* or false if check is exact */ PG_RETURN_BOOL(retval);}key is an element in the index andquery the value being looked up in the index. TheStrategyNumber parameter indicates which operator of your operator class is being applied — it matches one of the operator numbers in theCREATE OPERATOR CLASS command.query could vary with the operator, since it will be whatever type is on the righthand side of the operator, which might be different from the indexed data type appearing on the lefthand side. (The above code skeleton assumes that only one type is possible; if not, fetching thequery argument value would have to depend on the operator.) It is recommended that the SQL declaration of theconsistent function use the opclass's indexed data type for thequery argument, even though the actual type might be something else depending on the operator.unionCREATE OR REPLACE FUNCTION my_union(internal, internal)RETURNS storage_typeAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_union);Datummy_union(PG_FUNCTION_ARGS){ GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0); GISTENTRY *ent = entryvec->vector; data_type *out, *tmp, *old; int numranges, i = 0; numranges = entryvec->n; tmp = DatumGetDataType(ent[0].key); out = tmp; if (numranges == 1) { out = data_type_deep_copy(tmp); PG_RETURN_DATA_TYPE_P(out); } for (i = 1; i < numranges; i++) { old = out; tmp = DatumGetDataType(ent[i].key); out = my_union_implementation(out, tmp); } PG_RETURN_DATA_TYPE_P(out);}union(X, Y, Z) = union(union(X, Y), Z). It's easy enough to support data types where this is not the case, by implementing the proper union algorithm in thisGiST support method.union function must be a value of the index's storage type, whatever that is (it might or might not be different from the indexed column's type). Theunion function should return a pointer to newlypalloc()ed memory. You can't just return the input value as-is, even if there is no type change.union function's firstinternal argument is actually aGistEntryVector pointer. The second argument is a pointer to an integer variable, which can be ignored. (It used to be required that theunion function store the size of its result value into that variable, but this is no longer necessary.)compressCREATE OR REPLACE FUNCTION my_compress(internal)RETURNS internalAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_compress);Datummy_compress(PG_FUNCTION_ARGS){ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); GISTENTRY *retval; if (entry->leafkey) { /* replace entry->key with a compressed version */ compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type)); /* fill *compressed_data from entry->key ... */ retval = palloc(sizeof(GISTENTRY)); gistentryinit(*retval, PointerGetDatum(compressed_data), entry->rel, entry->page, entry->offset, FALSE); } else { /* typically we needn't do anything with non-leaf entries */ retval = entry; } PG_RETURN_POINTER(retval);}compressed_data_type to the specific type you're converting to in order to compress your leaf nodes, of course.decompresscompress method. Converts the index representation of the data item into a format that can be manipulated by the other GiST methods in the operator class.CREATE OR REPLACE FUNCTION my_decompress(internal)RETURNS internalAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_decompress);Datummy_decompress(PG_FUNCTION_ARGS){ PG_RETURN_POINTER(PG_GETARG_POINTER(0));}penaltypenalty in the tree. Values returned bypenalty should be non-negative. If a negative value is returned, it will be treated as zero.CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)RETURNS internalAS 'MODULE_PATHNAME'LANGUAGE C STRICT; -- in some cases penalty functions need not be strict
PG_FUNCTION_INFO_V1(my_penalty);Datummy_penalty(PG_FUNCTION_ARGS){ GISTENTRY *origentry = (GISTENTRY *) PG_GETARG_POINTER(0); GISTENTRY *newentry = (GISTENTRY *) PG_GETARG_POINTER(1); float *penalty = (float *) PG_GETARG_POINTER(2); data_type *orig = DatumGetDataType(origentry->key); data_type *new = DatumGetDataType(newentry->key); *penalty = my_penalty_implementation(orig, new); PG_RETURN_POINTER(penalty);}penalty function doesn't just return afloat result; instead it has to store the value at the location indicated by the third argument. The return value per se is ignored, though it's conventional to pass back the address of that argument.penalty function is crucial to good performance of the index. It'll get used at insertion time to determine which branch to follow when choosing where to add the new entry in the tree. At query time, the more balanced the index, the quicker the lookup.picksplitCREATE OR REPLACE FUNCTION my_picksplit(internal, internal)RETURNS internalAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_picksplit);Datummy_picksplit(PG_FUNCTION_ARGS){ GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0); GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1); OffsetNumber maxoff = entryvec->n - 1; GISTENTRY *ent = entryvec->vector; int i, nbytes; OffsetNumber *left, *right; data_type *tmp_union; data_type *unionL; data_type *unionR; GISTENTRY **raw_entryvec; maxoff = entryvec->n - 1; nbytes = (maxoff + 1) * sizeof(OffsetNumber); v->spl_left = (OffsetNumber *) palloc(nbytes); left = v->spl_left; v->spl_nleft = 0; v->spl_right = (OffsetNumber *) palloc(nbytes); right = v->spl_right; v->spl_nright = 0; unionL = NULL; unionR = NULL; /* Initialize the raw entry vector. */ raw_entryvec = (GISTENTRY **) malloc(entryvec->n * sizeof(void *)); for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i)) raw_entryvec[i] = &(entryvec->vector[i]); for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i)) { int real_index = raw_entryvec[i] - entryvec->vector; tmp_union = DatumGetDataType(entryvec->vector[real_index].key); Assert(tmp_union != NULL); /* * Choose where to put the index entries and update unionL and unionR * accordingly. Append the entries to either v->spl_left or * v->spl_right, and care about the counters. */ if (my_choice_is_left(unionL, curl, unionR, curr)) { if (unionL == NULL) unionL = tmp_union; else unionL = my_union_implementation(unionL, tmp_union); *left = real_index; ++left; ++(v->spl_nleft); } else { /* * Same on the right */ } } v->spl_ldatum = DataTypeGetDatum(unionL); v->spl_rdatum = DataTypeGetDatum(unionR); PG_RETURN_POINTER(v);}picksplit function's result is delivered by modifying the passed-inv structure. The return value per se is ignored, though it's conventional to pass back the address ofv.penalty, thepicksplit function is crucial to good performance of the index. Designing suitablepenalty andpicksplit implementations is where the challenge of implementing well-performingGiST indexes lies.sameCREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal)RETURNS internalAS 'MODULE_PATHNAME'LANGUAGE C STRICT;
PG_FUNCTION_INFO_V1(my_same);Datummy_same(PG_FUNCTION_ARGS){ prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0); prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1); bool *result = (bool *) PG_GETARG_POINTER(2); *result = my_eq(v1, v2); PG_RETURN_POINTER(result);}same function doesn't just return a Boolean result; instead it has to store the flag at the location indicated by the third argument. The return value per se is ignored, though it's conventional to pass back the address of that argument.distancep and a query valueq, this function determines the index entry's“distance” from the query value. This function must be supplied if the operator class contains any ordering operators. A query using the ordering operator will be implemented by returning index entries with the smallest“distance” values first, so the results must be consistent with the operator's semantics. For a leaf index entry the result just represents the distance to the index entry; for an internal tree node, the result must be the smallest distance that any child entry could have.CurrentMemoryContext will get reset after each tuple is processed. It is therefore not very important to worry about pfree'ing everything you palloc. However, in some cases it's useful for a support method to cache data across repeated calls. To do that, allocate the longer-lived data infcinfo->flinfo->fn_mcxt, and keep a pointer to it infcinfo->flinfo->fn_extra. Such data will survive for the life of the index operation (e.g., a single GiST index scan, index build, or index tuple insertion). Be careful to pfree the previous value when replacing afn_extra value, or the leak will accumulate for the duration of the operation.