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52.3. Implementation
There are seven methods that an index operator class for
GiST must provide. Correctness of the index is ensured
by proper implementation of the same, consistent
and union methods, while efficiency (size and speed) of the
index will depend on the penalty and picksplit
methods.
The remaining two methods are compress and
decompress, 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 follow PostgreSQL data type rules here,
see about varlena for variable sized data). If the tree's
internal data type exists at the SQL level, the STORAGE option
of the CREATE OPERATOR CLASS command can be used.
consistent-
Given an index entry p and a query value q, this function determines whether the index entry is "consistent" with the query; that is, could the predicate "indexed_column indexable_operator q" 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 is true, a recheck flag must also be returned. This indicates whether the predicate is certainly true or only possibly true. If recheck = false then the index has tested the predicate condition exactly, whereas if recheck = true the row is only a candidate match. In that case the system will automatically evaluate the indexable_operator against the actual row value to see if it is really a match. This convention allows GiST to support both lossless and lossy index structures.
The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal) RETURNS bool AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_consistent(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_consistent); Datum my_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); }Here, key is an element in the index and query the value being looked up in the index. The StrategyNumber parameter indicates which operator of your operator class is being applied — it matches one of the operator numbers in the CREATE OPERATOR CLASS command. Depending on what operators you have included in the class, the data type of query could vary with the operator, but the above skeleton assumes it doesn't.
union-
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.
The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_union(internal, internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_union(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_union); Datum my_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); }As you can see, in this skeleton we're dealing with a data type where 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 this GiST support method.
The
unionimplementation function should return a pointer to newlypalloc()ed memory. You can't just return whatever the input is. compress-
Converts the data item into a format suitable for physical storage in an index page.
The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_compress(internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_compress(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_compress); Datum my_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); }You have to adapt compressed_data_type to the specific type you're converting to in order to compress your leaf nodes, of course.
Depending on your needs, you could also need to care about compressing NULL values in there, storing for example (Datum) 0 like gist_circle_compress does.
decompress-
The reverse of the
compressmethod. Converts the index representation of the data item into a format that can be manipulated by the database.The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_decompress(internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_decompress(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_decompress); Datum my_decompress(PG_FUNCTION_ARGS) { PG_RETURN_POINTER(PG_GETARG_POINTER(0)); }The above skeleton is suitable for the case where no decompression is needed.
penalty-
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
penaltyin the tree.The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT; -- in some cases penalty functions need not be strict
And the matching code in the C module could then follow this skeleton:
Datum my_penalty(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_penalty); Datum my_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); }The
penaltyfunction 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. picksplit-
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.
The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_picksplit(internal, internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_picksplit(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_picksplit); Datum my_picksplit(PG_FUNCTION_ARGS) { GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0); OffsetNumber maxoff = entryvec->n - 1; GISTENTRY *ent = entryvec->vector; GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1); 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); }Like
penalty, thepicksplitfunction is crucial to good performance of the index. Designing suitablepenaltyandpicksplitimplementations is where the challenge of implementing well-performing GiST indexes lies. same-
Returns true if two index entries are identical, false otherwise.
The SQL declaration of the function must look like this:
CREATE OR REPLACE FUNCTION my_same(internal, internal, internal) RETURNS internal AS 'MODULE_PATHNAME' LANGUAGE C STRICT;
And the matching code in the C module could then follow this skeleton:
Datum my_same(PG_FUNCTION_ARGS); PG_FUNCTION_INFO_V1(my_same); Datum my_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); }For historical reasons, the
samefunction doesn't just return a Boolean result; instead it has to store the flag at the location indicated by the third argument.