-
+
GiST Indexes
- Some of the information here is derived from the University of California at
- Berkeley's GiST Indexing Project
+ Some of the information here is derived from the University of California
+ at Berkeley's GiST Indexing Project
+ Marcel Kornacker's thesis,
- Marcel Kornacker's thesis, Access Methods for Next-Generation Database Systems.
+ Access Methods for Next-Generation Database Systems.
implementation in
PostgreSQL is primarily
maintained by Teodor Sigaev and Oleg Bartunov, and there is more
information on their
difficult work. It was necessary to understand the inner workings of the
database, such as the lock manager and Write-Ahead Log. The
GiST interface has a high level of abstraction,
- requiring the access method implementer to only implement the semantics of
+ requiring the access method implementer only to implement the semantics of
the data type being accessed. The
GiST 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
(<, =, >),
and hash indexes only support equality queries.
-
+
So if you index, say, an image collection with a
PostgreSQL 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
?
+ 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 a
GiST based index, you could create
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 datatype 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
+ consistent
- Given a predicate p on a tree page, and a user
- query, q, this method will return false if it is
- certain that both p and q cannot
- be true for a given data item. For a true result, a
- recheck flag must also be returned; this indicates whether
- the predicate implies the query (recheck = false) or
- not (recheck = true).
+ 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
+ union
This method consolidates information in the tree. Given a set of
- entries, this function generates a new predicate that is true for all
- the entries.
+ 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
+
+
+ The union implementation function should return a
+ pointer to newly palloc()ed memory. You can't just
+ return whatever the input is.
- compress
+ 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
+ decompress
The reverse of the compress method. 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
+ penalty
Returns a value indicating the cost
of inserting the new
- entry into a particular branch of the tree. items will be inserted
+ entry into a particular branch of the tree. Items will be inserted
down the path of least penalty in 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 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.
+
- picksplit
+ picksplit
- When a 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.
+ 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, the picksplit function
+ is crucial to good performance of the index. Designing suitable
+ penalty and picksplit implementations
+ is where the challenge of implementing well-performing
- same
+ same
- Returns true if two entries are identical, false otherwise.
+ 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 same function doesn't
+ just return a boolean result; instead it has to store the flag
+ at the location indicated by the third argument.
R-Tree equivalent functionality for some of the built-in geometric data types
(see src/backend/access/gist/gistproc.c). The following
contrib modules also contain GiST
- operator classes:
+ operator classes:
-
+
btree_gist
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