1<!-- doc/src/sgml/gist.sgml --> 2 3<chapter id="gist"> 4<title>GiST Indexes</title> 5 6 <indexterm> 7 <primary>index</primary> 8 <secondary>GiST</secondary> 9 </indexterm> 10 11<sect1 id="gist-intro"> 12 <title>Introduction</title> 13 14 <para> 15 <acronym>GiST</acronym> stands for Generalized Search Tree. It is a 16 balanced, tree-structured access method, that acts as a base template in 17 which to implement arbitrary indexing schemes. B-trees, R-trees and many 18 other indexing schemes can be implemented in <acronym>GiST</acronym>. 19 </para> 20 21 <para> 22 One advantage of <acronym>GiST</acronym> is that it allows the development 23 of custom data types with the appropriate access methods, by 24 an expert in the domain of the data type, rather than a database expert. 25 </para> 26 27 <para> 28 Some of the information here is derived from the University of California 29 at Berkeley's GiST Indexing Project 30 <ulink url="http://gist.cs.berkeley.edu/">web site</ulink> and 31 Marcel Kornacker's thesis, 32 <ulink url="http://www.sai.msu.su/~megera/postgres/gist/papers/concurrency/access-methods-for-next-generation.pdf.gz"> 33 Access Methods for Next-Generation Database Systems</ulink>. 34 The <acronym>GiST</acronym> 35 implementation in <productname>PostgreSQL</productname> is primarily 36 maintained by Teodor Sigaev and Oleg Bartunov, and there is more 37 information on their 38 <ulink url="http://www.sai.msu.su/~megera/postgres/gist/">web site</ulink>. 39 </para> 40 41</sect1> 42 43<sect1 id="gist-builtin-opclasses"> 44 <title>Built-in Operator Classes</title> 45 46 <para> 47 The core <productname>PostgreSQL</productname> distribution 48 includes the <acronym>GiST</acronym> operator classes shown in 49 <xref linkend="gist-builtin-opclasses-table"/>. 50 (Some of the optional modules described in <xref linkend="contrib"/> 51 provide additional <acronym>GiST</acronym> operator classes.) 52 </para> 53 54 <table id="gist-builtin-opclasses-table"> 55 <title>Built-in <acronym>GiST</acronym> Operator Classes</title> 56 <tgroup cols="4"> 57 <thead> 58 <row> 59 <entry>Name</entry> 60 <entry>Indexed Data Type</entry> 61 <entry>Indexable Operators</entry> 62 <entry>Ordering Operators</entry> 63 </row> 64 </thead> 65 <tbody> 66 <row> 67 <entry><literal>box_ops</literal></entry> 68 <entry><type>box</type></entry> 69 <entry> 70 <literal>&&</literal> 71 <literal>&></literal> 72 <literal>&<</literal> 73 <literal>&<|</literal> 74 <literal>>></literal> 75 <literal><<</literal> 76 <literal><<|</literal> 77 <literal><@</literal> 78 <literal>@></literal> 79 <literal>@</literal> 80 <literal>|&></literal> 81 <literal>|>></literal> 82 <literal>~</literal> 83 <literal>~=</literal> 84 </entry> 85 <entry> 86 </entry> 87 </row> 88 <row> 89 <entry><literal>circle_ops</literal></entry> 90 <entry><type>circle</type></entry> 91 <entry> 92 <literal>&&</literal> 93 <literal>&></literal> 94 <literal>&<</literal> 95 <literal>&<|</literal> 96 <literal>>></literal> 97 <literal><<</literal> 98 <literal><<|</literal> 99 <literal><@</literal> 100 <literal>@></literal> 101 <literal>@</literal> 102 <literal>|&></literal> 103 <literal>|>></literal> 104 <literal>~</literal> 105 <literal>~=</literal> 106 </entry> 107 <entry> 108 <literal><-></literal> 109 </entry> 110 </row> 111 <row> 112 <entry><literal>inet_ops</literal></entry> 113 <entry><type>inet</type>, <type>cidr</type></entry> 114 <entry> 115 <literal>&&</literal> 116 <literal>>></literal> 117 <literal>>>=</literal> 118 <literal>></literal> 119 <literal>>=</literal> 120 <literal><></literal> 121 <literal><<</literal> 122 <literal><<=</literal> 123 <literal><</literal> 124 <literal><=</literal> 125 <literal>=</literal> 126 </entry> 127 <entry> 128 </entry> 129 </row> 130 <row> 131 <entry><literal>point_ops</literal></entry> 132 <entry><type>point</type></entry> 133 <entry> 134 <literal>>></literal> 135 <literal>>^</literal> 136 <literal><<</literal> 137 <literal><@</literal> 138 <literal><@</literal> 139 <literal><@</literal> 140 <literal><^</literal> 141 <literal>~=</literal> 142 </entry> 143 <entry> 144 <literal><-></literal> 145 </entry> 146 </row> 147 <row> 148 <entry><literal>poly_ops</literal></entry> 149 <entry><type>polygon</type></entry> 150 <entry> 151 <literal>&&</literal> 152 <literal>&></literal> 153 <literal>&<</literal> 154 <literal>&<|</literal> 155 <literal>>></literal> 156 <literal><<</literal> 157 <literal><<|</literal> 158 <literal><@</literal> 159 <literal>@></literal> 160 <literal>@</literal> 161 <literal>|&></literal> 162 <literal>|>></literal> 163 <literal>~</literal> 164 <literal>~=</literal> 165 </entry> 166 <entry> 167 <literal><-></literal> 168 </entry> 169 </row> 170 <row> 171 <entry><literal>range_ops</literal></entry> 172 <entry>any range type</entry> 173 <entry> 174 <literal>&&</literal> 175 <literal>&></literal> 176 <literal>&<</literal> 177 <literal>>></literal> 178 <literal><<</literal> 179 <literal><@</literal> 180 <literal>-|-</literal> 181 <literal>=</literal> 182 <literal>@></literal> 183 <literal>@></literal> 184 </entry> 185 <entry> 186 </entry> 187 </row> 188 <row> 189 <entry><literal>tsquery_ops</literal></entry> 190 <entry><type>tsquery</type></entry> 191 <entry> 192 <literal><@</literal> 193 <literal>@></literal> 194 </entry> 195 <entry> 196 </entry> 197 </row> 198 <row> 199 <entry><literal>tsvector_ops</literal></entry> 200 <entry><type>tsvector</type></entry> 201 <entry> 202 <literal>@@</literal> 203 </entry> 204 <entry> 205 </entry> 206 </row> 207 </tbody> 208 </tgroup> 209 </table> 210 211 <para> 212 For historical reasons, the <literal>inet_ops</literal> operator class is 213 not the default class for types <type>inet</type> and <type>cidr</type>. 214 To use it, mention the class name in <command>CREATE INDEX</command>, 215 for example 216<programlisting> 217CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops); 218</programlisting> 219 </para> 220 221</sect1> 222 223<sect1 id="gist-extensibility"> 224 <title>Extensibility</title> 225 226 <para> 227 Traditionally, implementing a new index access method meant a lot of 228 difficult work. It was necessary to understand the inner workings of the 229 database, such as the lock manager and Write-Ahead Log. The 230 <acronym>GiST</acronym> interface has a high level of abstraction, 231 requiring the access method implementer only to implement the semantics of 232 the data type being accessed. The <acronym>GiST</acronym> layer itself 233 takes care of concurrency, logging and searching the tree structure. 234 </para> 235 236 <para> 237 This extensibility should not be confused with the extensibility of the 238 other standard search trees in terms of the data they can handle. For 239 example, <productname>PostgreSQL</productname> supports extensible B-trees 240 and hash indexes. That means that you can use 241 <productname>PostgreSQL</productname> to build a B-tree or hash over any 242 data type you want. But B-trees only support range predicates 243 (<literal><</literal>, <literal>=</literal>, <literal>></literal>), 244 and hash indexes only support equality queries. 245 </para> 246 247 <para> 248 So if you index, say, an image collection with a 249 <productname>PostgreSQL</productname> B-tree, you can only issue queries 250 such as <quote>is imagex equal to imagey</quote>, <quote>is imagex less 251 than imagey</quote> and <quote>is imagex greater than imagey</quote>. 252 Depending on how you define <quote>equals</quote>, <quote>less than</quote> 253 and <quote>greater than</quote> in this context, this could be useful. 254 However, by using a <acronym>GiST</acronym> based index, you could create 255 ways to ask domain-specific questions, perhaps <quote>find all images of 256 horses</quote> or <quote>find all over-exposed images</quote>. 257 </para> 258 259 <para> 260 All it takes to get a <acronym>GiST</acronym> access method up and running 261 is to implement several user-defined methods, which define the behavior of 262 keys in the tree. Of course these methods have to be pretty fancy to 263 support fancy queries, but for all the standard queries (B-trees, 264 R-trees, etc.) they're relatively straightforward. In short, 265 <acronym>GiST</acronym> combines extensibility along with generality, code 266 reuse, and a clean interface. 267 </para> 268 269 <para> 270 There are five methods that an index operator class for 271 <acronym>GiST</acronym> must provide, and four that are optional. 272 Correctness of the index is ensured 273 by proper implementation of the <function>same</function>, <function>consistent</function> 274 and <function>union</function> methods, while efficiency (size and speed) of the 275 index will depend on the <function>penalty</function> and <function>picksplit</function> 276 methods. 277 Two optional methods are <function>compress</function> and 278 <function>decompress</function>, which allow an index to have internal tree data of 279 a different type than the data it indexes. The leaves are to be of the 280 indexed data type, while the other tree nodes can be of any C struct (but 281 you still have to follow <productname>PostgreSQL</productname> data type rules here, 282 see about <literal>varlena</literal> for variable sized data). If the tree's 283 internal data type exists at the SQL level, the <literal>STORAGE</literal> option 284 of the <command>CREATE OPERATOR CLASS</command> command can be used. 285 The optional eighth method is <function>distance</function>, which is needed 286 if the operator class wishes to support ordered scans (nearest-neighbor 287 searches). The optional ninth method <function>fetch</function> is needed if the 288 operator class wishes to support index-only scans, except when the 289 <function>compress</function> method is omitted. 290 </para> 291 292 <variablelist> 293 <varlistentry> 294 <term><function>consistent</function></term> 295 <listitem> 296 <para> 297 Given an index entry <literal>p</literal> and a query value <literal>q</literal>, 298 this function determines whether the index entry is 299 <quote>consistent</quote> with the query; that is, could the predicate 300 <quote><replaceable>indexed_column</replaceable> 301 <replaceable>indexable_operator</replaceable> <literal>q</literal></quote> be true for 302 any row represented by the index entry? For a leaf index entry this is 303 equivalent to testing the indexable condition, while for an internal 304 tree node this determines whether it is necessary to scan the subtree 305 of the index represented by the tree node. When the result is 306 <literal>true</literal>, a <literal>recheck</literal> flag must also be returned. 307 This indicates whether the predicate is certainly true or only possibly 308 true. If <literal>recheck</literal> = <literal>false</literal> then the index has 309 tested the predicate condition exactly, whereas if <literal>recheck</literal> 310 = <literal>true</literal> the row is only a candidate match. In that case the 311 system will automatically evaluate the 312 <replaceable>indexable_operator</replaceable> against the actual row value to see 313 if it is really a match. This convention allows 314 <acronym>GiST</acronym> to support both lossless and lossy index 315 structures. 316 </para> 317 318 <para> 319 The <acronym>SQL</acronym> declaration of the function must look like this: 320 321<programlisting> 322CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal) 323RETURNS bool 324AS 'MODULE_PATHNAME' 325LANGUAGE C STRICT; 326</programlisting> 327 328 And the matching code in the C module could then follow this skeleton: 329 330<programlisting> 331PG_FUNCTION_INFO_V1(my_consistent); 332 333Datum 334my_consistent(PG_FUNCTION_ARGS) 335{ 336 GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); 337 data_type *query = PG_GETARG_DATA_TYPE_P(1); 338 StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2); 339 /* Oid subtype = PG_GETARG_OID(3); */ 340 bool *recheck = (bool *) PG_GETARG_POINTER(4); 341 data_type *key = DatumGetDataType(entry->key); 342 bool retval; 343 344 /* 345 * determine return value as a function of strategy, key and query. 346 * 347 * Use GIST_LEAF(entry) to know where you're called in the index tree, 348 * which comes handy when supporting the = operator for example (you could 349 * check for non empty union() in non-leaf nodes and equality in leaf 350 * nodes). 351 */ 352 353 *recheck = true; /* or false if check is exact */ 354 355 PG_RETURN_BOOL(retval); 356} 357</programlisting> 358 359 Here, <varname>key</varname> is an element in the index and <varname>query</varname> 360 the value being looked up in the index. The <literal>StrategyNumber</literal> 361 parameter indicates which operator of your operator class is being 362 applied — it matches one of the operator numbers in the 363 <command>CREATE OPERATOR CLASS</command> command. 364 </para> 365 366 <para> 367 Depending on which operators you have included in the class, the data 368 type of <varname>query</varname> could vary with the operator, since it will 369 be whatever type is on the righthand side of the operator, which might 370 be different from the indexed data type appearing on the lefthand side. 371 (The above code skeleton assumes that only one type is possible; if 372 not, fetching the <varname>query</varname> argument value would have to depend 373 on the operator.) It is recommended that the SQL declaration of 374 the <function>consistent</function> function use the opclass's indexed data 375 type for the <varname>query</varname> argument, even though the actual type 376 might be something else depending on the operator. 377 </para> 378 379 </listitem> 380 </varlistentry> 381 382 <varlistentry> 383 <term><function>union</function></term> 384 <listitem> 385 <para> 386 This method consolidates information in the tree. Given a set of 387 entries, this function generates a new index entry that represents 388 all the given entries. 389 </para> 390 391 <para> 392 The <acronym>SQL</acronym> declaration of the function must look like this: 393 394<programlisting> 395CREATE OR REPLACE FUNCTION my_union(internal, internal) 396RETURNS storage_type 397AS 'MODULE_PATHNAME' 398LANGUAGE C STRICT; 399</programlisting> 400 401 And the matching code in the C module could then follow this skeleton: 402 403<programlisting> 404PG_FUNCTION_INFO_V1(my_union); 405 406Datum 407my_union(PG_FUNCTION_ARGS) 408{ 409 GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0); 410 GISTENTRY *ent = entryvec->vector; 411 data_type *out, 412 *tmp, 413 *old; 414 int numranges, 415 i = 0; 416 417 numranges = entryvec->n; 418 tmp = DatumGetDataType(ent[0].key); 419 out = tmp; 420 421 if (numranges == 1) 422 { 423 out = data_type_deep_copy(tmp); 424 425 PG_RETURN_DATA_TYPE_P(out); 426 } 427 428 for (i = 1; i < numranges; i++) 429 { 430 old = out; 431 tmp = DatumGetDataType(ent[i].key); 432 out = my_union_implementation(out, tmp); 433 } 434 435 PG_RETURN_DATA_TYPE_P(out); 436} 437</programlisting> 438 </para> 439 440 <para> 441 As you can see, in this skeleton we're dealing with a data type 442 where <literal>union(X, Y, Z) = union(union(X, Y), Z)</literal>. It's easy 443 enough to support data types where this is not the case, by 444 implementing the proper union algorithm in this 445 <acronym>GiST</acronym> support method. 446 </para> 447 448 <para> 449 The result of the <function>union</function> function must be a value of the 450 index's storage type, whatever that is (it might or might not be 451 different from the indexed column's type). The <function>union</function> 452 function should return a pointer to newly <function>palloc()</function>ed 453 memory. You can't just return the input value as-is, even if there is 454 no type change. 455 </para> 456 457 <para> 458 As shown above, the <function>union</function> function's 459 first <type>internal</type> argument is actually 460 a <structname>GistEntryVector</structname> pointer. The second argument is a 461 pointer to an integer variable, which can be ignored. (It used to be 462 required that the <function>union</function> function store the size of its 463 result value into that variable, but this is no longer necessary.) 464 </para> 465 </listitem> 466 </varlistentry> 467 468 <varlistentry> 469 <term><function>compress</function></term> 470 <listitem> 471 <para> 472 Converts a data item into a format suitable for physical storage in 473 an index page. 474 If the <function>compress</function> method is omitted, data items are stored 475 in the index without modification. 476 </para> 477 478 <para> 479 The <acronym>SQL</acronym> declaration of the function must look like this: 480 481<programlisting> 482CREATE OR REPLACE FUNCTION my_compress(internal) 483RETURNS internal 484AS 'MODULE_PATHNAME' 485LANGUAGE C STRICT; 486</programlisting> 487 488 And the matching code in the C module could then follow this skeleton: 489 490<programlisting> 491PG_FUNCTION_INFO_V1(my_compress); 492 493Datum 494my_compress(PG_FUNCTION_ARGS) 495{ 496 GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); 497 GISTENTRY *retval; 498 499 if (entry->leafkey) 500 { 501 /* replace entry->key with a compressed version */ 502 compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type)); 503 504 /* fill *compressed_data from entry->key ... */ 505 506 retval = palloc(sizeof(GISTENTRY)); 507 gistentryinit(*retval, PointerGetDatum(compressed_data), 508 entry->rel, entry->page, entry->offset, FALSE); 509 } 510 else 511 { 512 /* typically we needn't do anything with non-leaf entries */ 513 retval = entry; 514 } 515 516 PG_RETURN_POINTER(retval); 517} 518</programlisting> 519 </para> 520 521 <para> 522 You have to adapt <replaceable>compressed_data_type</replaceable> to the specific 523 type you're converting to in order to compress your leaf nodes, of 524 course. 525 </para> 526 </listitem> 527 </varlistentry> 528 529 <varlistentry> 530 <term><function>decompress</function></term> 531 <listitem> 532 <para> 533 Converts the stored representation of a data item into a format that 534 can be manipulated by the other GiST methods in the operator class. 535 If the <function>decompress</function> method is omitted, it is assumed that 536 the other GiST methods can work directly on the stored data format. 537 (<function>decompress</function> is not necessarily the reverse of 538 the <function>compress</function> method; in particular, 539 if <function>compress</function> is lossy then it's impossible 540 for <function>decompress</function> to exactly reconstruct the original 541 data. <function>decompress</function> is not necessarily equivalent 542 to <function>fetch</function>, either, since the other GiST methods might not 543 require full reconstruction of the data.) 544 </para> 545 546 <para> 547 The <acronym>SQL</acronym> declaration of the function must look like this: 548 549<programlisting> 550CREATE OR REPLACE FUNCTION my_decompress(internal) 551RETURNS internal 552AS 'MODULE_PATHNAME' 553LANGUAGE C STRICT; 554</programlisting> 555 556 And the matching code in the C module could then follow this skeleton: 557 558<programlisting> 559PG_FUNCTION_INFO_V1(my_decompress); 560 561Datum 562my_decompress(PG_FUNCTION_ARGS) 563{ 564 PG_RETURN_POINTER(PG_GETARG_POINTER(0)); 565} 566</programlisting> 567 568 The above skeleton is suitable for the case where no decompression 569 is needed. (But, of course, omitting the method altogether is even 570 easier, and is recommended in such cases.) 571 </para> 572 </listitem> 573 </varlistentry> 574 575 <varlistentry> 576 <term><function>penalty</function></term> 577 <listitem> 578 <para> 579 Returns a value indicating the <quote>cost</quote> of inserting the new 580 entry into a particular branch of the tree. Items will be inserted 581 down the path of least <function>penalty</function> in the tree. 582 Values returned by <function>penalty</function> should be non-negative. 583 If a negative value is returned, it will be treated as zero. 584 </para> 585 586 <para> 587 The <acronym>SQL</acronym> declaration of the function must look like this: 588 589<programlisting> 590CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal) 591RETURNS internal 592AS 'MODULE_PATHNAME' 593LANGUAGE C STRICT; -- in some cases penalty functions need not be strict 594</programlisting> 595 596 And the matching code in the C module could then follow this skeleton: 597 598<programlisting> 599PG_FUNCTION_INFO_V1(my_penalty); 600 601Datum 602my_penalty(PG_FUNCTION_ARGS) 603{ 604 GISTENTRY *origentry = (GISTENTRY *) PG_GETARG_POINTER(0); 605 GISTENTRY *newentry = (GISTENTRY *) PG_GETARG_POINTER(1); 606 float *penalty = (float *) PG_GETARG_POINTER(2); 607 data_type *orig = DatumGetDataType(origentry->key); 608 data_type *new = DatumGetDataType(newentry->key); 609 610 *penalty = my_penalty_implementation(orig, new); 611 PG_RETURN_POINTER(penalty); 612} 613</programlisting> 614 615 For historical reasons, the <function>penalty</function> function doesn't 616 just return a <type>float</type> result; instead it has to store the value 617 at the location indicated by the third argument. The return 618 value per se is ignored, though it's conventional to pass back the 619 address of that argument. 620 </para> 621 622 <para> 623 The <function>penalty</function> function is crucial to good performance of 624 the index. It'll get used at insertion time to determine which branch 625 to follow when choosing where to add the new entry in the tree. At 626 query time, the more balanced the index, the quicker the lookup. 627 </para> 628 </listitem> 629 </varlistentry> 630 631 <varlistentry> 632 <term><function>picksplit</function></term> 633 <listitem> 634 <para> 635 When an index page split is necessary, this function decides which 636 entries on the page are to stay on the old page, and which are to move 637 to the new page. 638 </para> 639 640 <para> 641 The <acronym>SQL</acronym> declaration of the function must look like this: 642 643<programlisting> 644CREATE OR REPLACE FUNCTION my_picksplit(internal, internal) 645RETURNS internal 646AS 'MODULE_PATHNAME' 647LANGUAGE C STRICT; 648</programlisting> 649 650 And the matching code in the C module could then follow this skeleton: 651 652<programlisting> 653PG_FUNCTION_INFO_V1(my_picksplit); 654 655Datum 656my_picksplit(PG_FUNCTION_ARGS) 657{ 658 GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0); 659 GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1); 660 OffsetNumber maxoff = entryvec->n - 1; 661 GISTENTRY *ent = entryvec->vector; 662 int i, 663 nbytes; 664 OffsetNumber *left, 665 *right; 666 data_type *tmp_union; 667 data_type *unionL; 668 data_type *unionR; 669 GISTENTRY **raw_entryvec; 670 671 maxoff = entryvec->n - 1; 672 nbytes = (maxoff + 1) * sizeof(OffsetNumber); 673 674 v->spl_left = (OffsetNumber *) palloc(nbytes); 675 left = v->spl_left; 676 v->spl_nleft = 0; 677 678 v->spl_right = (OffsetNumber *) palloc(nbytes); 679 right = v->spl_right; 680 v->spl_nright = 0; 681 682 unionL = NULL; 683 unionR = NULL; 684 685 /* Initialize the raw entry vector. */ 686 raw_entryvec = (GISTENTRY **) malloc(entryvec->n * sizeof(void *)); 687 for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i)) 688 raw_entryvec[i] = &(entryvec->vector[i]); 689 690 for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i)) 691 { 692 int real_index = raw_entryvec[i] - entryvec->vector; 693 694 tmp_union = DatumGetDataType(entryvec->vector[real_index].key); 695 Assert(tmp_union != NULL); 696 697 /* 698 * Choose where to put the index entries and update unionL and unionR 699 * accordingly. Append the entries to either v->spl_left or 700 * v->spl_right, and care about the counters. 701 */ 702 703 if (my_choice_is_left(unionL, curl, unionR, curr)) 704 { 705 if (unionL == NULL) 706 unionL = tmp_union; 707 else 708 unionL = my_union_implementation(unionL, tmp_union); 709 710 *left = real_index; 711 ++left; 712 ++(v->spl_nleft); 713 } 714 else 715 { 716 /* 717 * Same on the right 718 */ 719 } 720 } 721 722 v->spl_ldatum = DataTypeGetDatum(unionL); 723 v->spl_rdatum = DataTypeGetDatum(unionR); 724 PG_RETURN_POINTER(v); 725} 726</programlisting> 727 728 Notice that the <function>picksplit</function> function's result is delivered 729 by modifying the passed-in <structname>v</structname> structure. The return 730 value per se is ignored, though it's conventional to pass back the 731 address of <structname>v</structname>. 732 </para> 733 734 <para> 735 Like <function>penalty</function>, the <function>picksplit</function> function 736 is crucial to good performance of the index. Designing suitable 737 <function>penalty</function> and <function>picksplit</function> implementations 738 is where the challenge of implementing well-performing 739 <acronym>GiST</acronym> indexes lies. 740 </para> 741 </listitem> 742 </varlistentry> 743 744 <varlistentry> 745 <term><function>same</function></term> 746 <listitem> 747 <para> 748 Returns true if two index entries are identical, false otherwise. 749 (An <quote>index entry</quote> is a value of the index's storage type, 750 not necessarily the original indexed column's type.) 751 </para> 752 753 <para> 754 The <acronym>SQL</acronym> declaration of the function must look like this: 755 756<programlisting> 757CREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal) 758RETURNS internal 759AS 'MODULE_PATHNAME' 760LANGUAGE C STRICT; 761</programlisting> 762 763 And the matching code in the C module could then follow this skeleton: 764 765<programlisting> 766PG_FUNCTION_INFO_V1(my_same); 767 768Datum 769my_same(PG_FUNCTION_ARGS) 770{ 771 prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0); 772 prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1); 773 bool *result = (bool *) PG_GETARG_POINTER(2); 774 775 *result = my_eq(v1, v2); 776 PG_RETURN_POINTER(result); 777} 778</programlisting> 779 780 For historical reasons, the <function>same</function> function doesn't 781 just return a Boolean result; instead it has to store the flag 782 at the location indicated by the third argument. The return 783 value per se is ignored, though it's conventional to pass back the 784 address of that argument. 785 </para> 786 </listitem> 787 </varlistentry> 788 789 <varlistentry> 790 <term><function>distance</function></term> 791 <listitem> 792 <para> 793 Given an index entry <literal>p</literal> and a query value <literal>q</literal>, 794 this function determines the index entry's 795 <quote>distance</quote> from the query value. This function must be 796 supplied if the operator class contains any ordering operators. 797 A query using the ordering operator will be implemented by returning 798 index entries with the smallest <quote>distance</quote> values first, 799 so the results must be consistent with the operator's semantics. 800 For a leaf index entry the result just represents the distance to 801 the index entry; for an internal tree node, the result must be the 802 smallest distance that any child entry could have. 803 </para> 804 805 <para> 806 The <acronym>SQL</acronym> declaration of the function must look like this: 807 808<programlisting> 809CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal) 810RETURNS float8 811AS 'MODULE_PATHNAME' 812LANGUAGE C STRICT; 813</programlisting> 814 815 And the matching code in the C module could then follow this skeleton: 816 817<programlisting> 818PG_FUNCTION_INFO_V1(my_distance); 819 820Datum 821my_distance(PG_FUNCTION_ARGS) 822{ 823 GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); 824 data_type *query = PG_GETARG_DATA_TYPE_P(1); 825 StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2); 826 /* Oid subtype = PG_GETARG_OID(3); */ 827 /* bool *recheck = (bool *) PG_GETARG_POINTER(4); */ 828 data_type *key = DatumGetDataType(entry->key); 829 double retval; 830 831 /* 832 * determine return value as a function of strategy, key and query. 833 */ 834 835 PG_RETURN_FLOAT8(retval); 836} 837</programlisting> 838 839 The arguments to the <function>distance</function> function are identical to 840 the arguments of the <function>consistent</function> function. 841 </para> 842 843 <para> 844 Some approximation is allowed when determining the distance, so long 845 as the result is never greater than the entry's actual distance. Thus, 846 for example, distance to a bounding box is usually sufficient in 847 geometric applications. For an internal tree node, the distance 848 returned must not be greater than the distance to any of the child 849 nodes. If the returned distance is not exact, the function must set 850 <literal>*recheck</literal> to true. (This is not necessary for internal tree 851 nodes; for them, the calculation is always assumed to be inexact.) In 852 this case the executor will calculate the accurate distance after 853 fetching the tuple from the heap, and reorder the tuples if necessary. 854 </para> 855 856 <para> 857 If the distance function returns <literal>*recheck = true</literal> for any 858 leaf node, the original ordering operator's return type must 859 be <type>float8</type> or <type>float4</type>, and the distance function's 860 result values must be comparable to those of the original ordering 861 operator, since the executor will sort using both distance function 862 results and recalculated ordering-operator results. Otherwise, the 863 distance function's result values can be any finite <type>float8</type> 864 values, so long as the relative order of the result values matches the 865 order returned by the ordering operator. (Infinity and minus infinity 866 are used internally to handle cases such as nulls, so it is not 867 recommended that <function>distance</function> functions return these values.) 868 </para> 869 870 </listitem> 871 </varlistentry> 872 873 <varlistentry> 874 <term><function>fetch</function></term> 875 <listitem> 876 <para> 877 Converts the compressed index representation of a data item into the 878 original data type, for index-only scans. The returned data must be an 879 exact, non-lossy copy of the originally indexed value. 880 </para> 881 882 <para> 883 The <acronym>SQL</acronym> declaration of the function must look like this: 884 885<programlisting> 886CREATE OR REPLACE FUNCTION my_fetch(internal) 887RETURNS internal 888AS 'MODULE_PATHNAME' 889LANGUAGE C STRICT; 890</programlisting> 891 892 The argument is a pointer to a <structname>GISTENTRY</structname> struct. On 893 entry, its <structfield>key</structfield> field contains a non-NULL leaf datum in 894 compressed form. The return value is another <structname>GISTENTRY</structname> 895 struct, whose <structfield>key</structfield> field contains the same datum in its 896 original, uncompressed form. If the opclass's compress function does 897 nothing for leaf entries, the <function>fetch</function> method can return the 898 argument as-is. Or, if the opclass does not have a compress function, 899 the <function>fetch</function> method can be omitted as well, since it would 900 necessarily be a no-op. 901 </para> 902 903 <para> 904 The matching code in the C module could then follow this skeleton: 905 906<programlisting> 907PG_FUNCTION_INFO_V1(my_fetch); 908 909Datum 910my_fetch(PG_FUNCTION_ARGS) 911{ 912 GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0); 913 input_data_type *in = DatumGetPointer(entry->key); 914 fetched_data_type *fetched_data; 915 GISTENTRY *retval; 916 917 retval = palloc(sizeof(GISTENTRY)); 918 fetched_data = palloc(sizeof(fetched_data_type)); 919 920 /* 921 * Convert 'fetched_data' into the a Datum of the original datatype. 922 */ 923 924 /* fill *retval from fetched_data. */ 925 gistentryinit(*retval, PointerGetDatum(converted_datum), 926 entry->rel, entry->page, entry->offset, FALSE); 927 928 PG_RETURN_POINTER(retval); 929} 930</programlisting> 931 </para> 932 933 <para> 934 If the compress method is lossy for leaf entries, the operator class 935 cannot support index-only scans, and must not define 936 a <function>fetch</function> function. 937 </para> 938 939 </listitem> 940 </varlistentry> 941 </variablelist> 942 943 <para> 944 All the GiST support methods are normally called in short-lived memory 945 contexts; that is, <varname>CurrentMemoryContext</varname> will get reset after 946 each tuple is processed. It is therefore not very important to worry about 947 pfree'ing everything you palloc. However, in some cases it's useful for a 948 support method to cache data across repeated calls. To do that, allocate 949 the longer-lived data in <literal>fcinfo->flinfo->fn_mcxt</literal>, and 950 keep a pointer to it in <literal>fcinfo->flinfo->fn_extra</literal>. Such 951 data will survive for the life of the index operation (e.g., a single GiST 952 index scan, index build, or index tuple insertion). Be careful to pfree 953 the previous value when replacing a <literal>fn_extra</literal> value, or the leak 954 will accumulate for the duration of the operation. 955 </para> 956 957</sect1> 958 959<sect1 id="gist-implementation"> 960 <title>Implementation</title> 961 962 <sect2 id="gist-buffering-build"> 963 <title>GiST Buffering Build</title> 964 <para> 965 Building large GiST indexes by simply inserting all the tuples tends to be 966 slow, because if the index tuples are scattered across the index and the 967 index is large enough to not fit in cache, the insertions need to perform 968 a lot of random I/O. Beginning in version 9.2, PostgreSQL supports a more 969 efficient method to build GiST indexes based on buffering, which can 970 dramatically reduce the number of random I/Os needed for non-ordered data 971 sets. For well-ordered data sets the benefit is smaller or non-existent, 972 because only a small number of pages receive new tuples at a time, and 973 those pages fit in cache even if the index as whole does not. 974 </para> 975 976 <para> 977 However, buffering index build needs to call the <function>penalty</function> 978 function more often, which consumes some extra CPU resources. Also, the 979 buffers used in the buffering build need temporary disk space, up to 980 the size of the resulting index. Buffering can also influence the quality 981 of the resulting index, in both positive and negative directions. That 982 influence depends on various factors, like the distribution of the input 983 data and the operator class implementation. 984 </para> 985 986 <para> 987 By default, a GiST index build switches to the buffering method when the 988 index size reaches <xref linkend="guc-effective-cache-size"/>. It can 989 be manually turned on or off by the <literal>buffering</literal> parameter 990 to the CREATE INDEX command. The default behavior is good for most cases, 991 but turning buffering off might speed up the build somewhat if the input 992 data is ordered. 993 </para> 994 995 </sect2> 996</sect1> 997 998<sect1 id="gist-examples"> 999 <title>Examples</title> 1000 1001 <para> 1002 The <productname>PostgreSQL</productname> source distribution includes 1003 several examples of index methods implemented using 1004 <acronym>GiST</acronym>. The core system currently provides text search 1005 support (indexing for <type>tsvector</type> and <type>tsquery</type>) as well as 1006 R-Tree equivalent functionality for some of the built-in geometric data types 1007 (see <filename>src/backend/access/gist/gistproc.c</filename>). The following 1008 <filename>contrib</filename> modules also contain <acronym>GiST</acronym> 1009 operator classes: 1010 1011 <variablelist> 1012 <varlistentry> 1013 <term><filename>btree_gist</filename></term> 1014 <listitem> 1015 <para>B-tree equivalent functionality for several data types</para> 1016 </listitem> 1017 </varlistentry> 1018 1019 <varlistentry> 1020 <term><filename>cube</filename></term> 1021 <listitem> 1022 <para>Indexing for multidimensional cubes</para> 1023 </listitem> 1024 </varlistentry> 1025 1026 <varlistentry> 1027 <term><filename>hstore</filename></term> 1028 <listitem> 1029 <para>Module for storing (key, value) pairs</para> 1030 </listitem> 1031 </varlistentry> 1032 1033 <varlistentry> 1034 <term><filename>intarray</filename></term> 1035 <listitem> 1036 <para>RD-Tree for one-dimensional array of int4 values</para> 1037 </listitem> 1038 </varlistentry> 1039 1040 <varlistentry> 1041 <term><filename>ltree</filename></term> 1042 <listitem> 1043 <para>Indexing for tree-like structures</para> 1044 </listitem> 1045 </varlistentry> 1046 1047 <varlistentry> 1048 <term><filename>pg_trgm</filename></term> 1049 <listitem> 1050 <para>Text similarity using trigram matching</para> 1051 </listitem> 1052 </varlistentry> 1053 1054 <varlistentry> 1055 <term><filename>seg</filename></term> 1056 <listitem> 1057 <para>Indexing for <quote>float ranges</quote></para> 1058 </listitem> 1059 </varlistentry> 1060 </variablelist> 1061 </para> 1062 1063</sect1> 1064 1065</chapter> 1066