Datasets:
File size: 37,743 Bytes
6493056 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 | <!DOCTYPE html>
<html data-theme="light" lang="en"><head>
<title>PostgreSQL: Documentation: 18: F.35. pg_trgm — support for similarity of text using trigram matching</title>
<meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport"/>
<meta content="text/xhtml; charset=utf-8" http-equiv="Content-Type"/>
<meta content="#336791" name="theme-color"/>
<meta content="The PostgreSQL Global Development Group" name="copyright"/>
<meta content="/docs/18/pgtrgm.html" property="og:url"/>
<meta content="article" property="og:type"/>
<meta content="2026-02-26T14:07:09" property="article:published_time"/>
<meta content="/media/img/about/press/elephant.png" property="og:image"/>
<meta content="F.35. pg_trgm — support for similarity of text using trigram matching" property="og:title"/>
<meta content="F.35.&nbsp;pg_trgm — support for similarity of text using trigram matching # F.35.1. Trigram (or Trigraph) Concepts F.35.2. Functions and Operators …" property="og:description"/>
<meta content="PostgreSQL Documentation" property="og:site_name"/>
<link href="/media/css/fontawesome.css?54a05eb2" rel="stylesheet"/>
<link href="/media/css/bootstrap-4.4.1.min.css" rel="stylesheet"/>
<link href="/favicon.ico" rel="shortcut icon"/>
<link href="/dyncss/base.css?54a05eb2" rel="stylesheet" type="text/css"/>
<script src="/media/js/theme.js?54a05eb2"></script>
<link href="/docs/current/pgtrgm.html" rel="canonical"/>
</head>
<body>
<div class="container-fluid">
<div class="row justify-content-md-center">
<div class="col">
<!-- Header -->
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand p-0" href="/">
<img alt="PostgreSQL Elephant Logo" class="logo" src="/media/img/about/press/elephant.png"/>
</a>
<input aria-controls="pgNavbar" aria-expanded="false" aria-label="Toggle navigation" id="navbar-toggler" type="checkbox"/>
<label class="navbar-toggler" for="navbar-toggler" id="navbar-toggler-label" tabindex="0"><span class="navbar-toggler-icon"></span></label>
<div class="navbar-collapse" id="pgNavbar">
<ul class="navbar-nav mr-auto">
<li class="nav-item p-2"><a href="/" title="Home">Home</a></li>
<li class="nav-item p-2"><a href="/about/" title="About">About</a></li>
<li class="nav-item p-2"><a href="/download/" title="Download">Download</a></li>
<li class="nav-item p-2"><a href="/docs/" title="Documentation">Documentation</a></li>
<li class="nav-item p-2"><a href="/community/" title="Community">Community</a></li>
<li class="nav-item p-2"><a href="/developer/" title="Developers">Developers</a></li>
<li class="nav-item p-2"><a href="/support/" title="Support">Support</a></li>
<li class="nav-item p-2"><a href="/about/donate/" title="Donate">Donate</a></li>
<li class="nav-item p-2"><a href="/account/" title="Your account">Your account</a></li>
</ul>
<form action="/search/" method="get" role="search">
<div class="input-group">
<input accesskey="s" class="form-control" id="q" maxlength="255" name="q" placeholder="Search for..." size="20" type="text"/>
<span class="input-group-btn">
<button class="btn btn-default" type="submit"><i class="fas fa-search"></i></button>
</span>
</div><!-- /input-group -->
</form>
<form class="form-inline" id="form-theme">
<button class="btn btn-default ml-1" id="btn-theme" type="button"><i class="fas fa-moon"></i></button>
</form>
</div>
</nav>
</div>
</div>
<div class="row justify-content-center pg-shout-box">
<div class="col text-white text-center">February 26, 2026: <a href="/about/news/postgresql-183-179-1613-1517-and-1422-released-3246/">
PostgreSQL 18.3, 17.9, 16.13, 15.17, and 14.22 Released!
</a>
</div>
</div>
</div>
<div class="container-fluid margin">
<div class="row">
<div class="col-11" id="pgContentWrap">
<div class="row">
<div class="col-md-6 mb-2">
<div class="row">
<div class="col">
<div>
<a href="/docs/" title="Documentation">Documentation</a> → <a href="/docs/18/index.html">PostgreSQL 18</a>
</div>
</div>
</div>
<div class="row">
<div class="col">
Supported Versions:
<a class="docs-version-selected" href="/docs/current/pgtrgm.html" title="PostgreSQL 18 - F.35. pg_trgm — support for similarity of text using trigram matching">Current</a>
(<a class="docs-version-selected" href="/docs/18/pgtrgm.html" title="PostgreSQL 18 - F.35. pg_trgm — support for similarity of text using trigram matching">18</a>)
/
<a href="/docs/17/pgtrgm.html" title="PostgreSQL 17 - F.35. pg_trgm — support for similarity of text using trigram matching">17</a>
/
<a href="/docs/16/pgtrgm.html" title="PostgreSQL 16 - F.35. pg_trgm — support for similarity of text using trigram matching">16</a>
/
<a href="/docs/15/pgtrgm.html" title="PostgreSQL 15 - F.35. pg_trgm — support for similarity of text using trigram matching">15</a>
/
<a href="/docs/14/pgtrgm.html" title="PostgreSQL 14 - F.35. pg_trgm — support for similarity of text using trigram matching">14</a>
</div>
</div>
<div class="row">
<div class="col">
Development Versions:
<a href="/docs/devel/pgtrgm.html" rel="nofollow" title="PostgreSQL devel - F.35. pg_trgm — support for similarity of text using trigram matching">devel</a>
</div>
</div>
<div class="row">
<div class="col-12">
Unsupported versions:
<a href="/docs/13/pgtrgm.html" rel="nofollow" title="PostgreSQL 13 - F.35. pg_trgm — support for similarity of text using trigram matching">13</a>
/
<a href="/docs/12/pgtrgm.html" rel="nofollow" title="PostgreSQL 12 - F.35. pg_trgm — support for similarity of text using trigram matching">12</a>
/
<a href="/docs/11/pgtrgm.html" rel="nofollow" title="PostgreSQL 11 - F.35. pg_trgm — support for similarity of text using trigram matching">11</a>
/
<a href="/docs/10/pgtrgm.html" rel="nofollow" title="PostgreSQL 10 - F.35. pg_trgm — support for similarity of text using trigram matching">10</a>
/
<a href="/docs/9.6/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.6 - F.35. pg_trgm — support for similarity of text using trigram matching">9.6</a>
/
<a href="/docs/9.5/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.5 - F.35. pg_trgm — support for similarity of text using trigram matching">9.5</a>
/
<a href="/docs/9.4/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.4 - F.35. pg_trgm — support for similarity of text using trigram matching">9.4</a>
/
<a href="/docs/9.3/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.3 - F.35. pg_trgm — support for similarity of text using trigram matching">9.3</a>
/
<a href="/docs/9.2/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.2 - F.35. pg_trgm — support for similarity of text using trigram matching">9.2</a>
/
<a href="/docs/9.1/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.1 - F.35. pg_trgm — support for similarity of text using trigram matching">9.1</a>
/
<a href="/docs/9.0/pgtrgm.html" rel="nofollow" title="PostgreSQL 9.0 - F.35. pg_trgm — support for similarity of text using trigram matching">9.0</a>
/
<a href="/docs/8.4/pgtrgm.html" rel="nofollow" title="PostgreSQL 8.4 - F.35. pg_trgm — support for similarity of text using trigram matching">8.4</a>
/
<a href="/docs/8.3/pgtrgm.html" rel="nofollow" title="PostgreSQL 8.3 - F.35. pg_trgm — support for similarity of text using trigram matching">8.3</a>
</div>
</div>
</div>
<div class="col-md-6 col-lg-5 offset-lg-1">
<form action="/search/" id="docSearchForm" method="get" role="search">
<input name="u" type="hidden" value="/docs/18/"/>
<div class="input-group">
<input class="form-control" id="q" name="q" placeholder="Search the documentation for..." type="text"/>
<span class="input-group-btn">
<button class="btn btn-default" type="submit"><i class="fas fa-search"></i></button>
</span>
</div><!-- /input-group -->
</form>
</div>
</div>
<div id="docContent">
<div class="navheader">
<table summary="Navigation header" width="100%">
<tbody><tr>
<th align="center" colspan="5">F.35. pg_trgm — support for similarity of text using trigram matching</th>
</tr>
<tr>
<td align="left" width="10%"><a accesskey="p" href="/docs/current/pgsurgery.html" title="F.34. pg_surgery — perform low-level surgery on relation data">Prev</a> </td>
<td align="left" width="10%"><a accesskey="u" href="/docs/current/contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td>
<th align="center" width="60%">Appendix F. Additional Supplied Modules and Extensions</th>
<td align="right" width="10%"><a accesskey="h" href="/docs/current/index.html" title="PostgreSQL 18.3 Documentation">Home</a></td>
<td align="right" width="10%"> <a accesskey="n" href="/docs/current/pgvisibility.html" title="F.36. pg_visibility — visibility map information and utilities">Next</a></td>
</tr>
</tbody></table>
<hr/>
</div>
<div class="sect1" id="PGTRGM">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">F.35. pg_trgm — support for similarity of text using trigram matching <a class="id_link" href="#PGTRGM">#</a></h2>
</div>
</div>
</div>
<div class="toc">
<dl class="toc">
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-CONCEPTS">F.35.1. Trigram (or Trigraph) Concepts</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-FUNCS-OPS">F.35.2. Functions and Operators</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-GUC">F.35.3. GUC Parameters</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-INDEX">F.35.4. Index Support</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-TEXT-SEARCH">F.35.5. Text Search Integration</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-REFERENCES">F.35.6. References</a></span></dt>
<dt><span class="sect2"><a href="/docs/current/pgtrgm.html#PGTRGM-AUTHORS">F.35.7. Authors</a></span></dt>
</dl>
</div><a class="indexterm" id="id-1.11.7.45.2" name="id-1.11.7.45.2"></a>
<p>The <code class="filename">pg_trgm</code> module provides functions and operators for determining the similarity of alphanumeric text based on trigram matching, as well as index operator classes that support fast searching for similar strings.</p>
<p>This module is considered <span class="quote">“<span class="quote">trusted</span>”</span>, that is, it can be installed by non-superusers who have <code class="literal">CREATE</code> privilege on the current database.</p>
<div class="sect2" id="PGTRGM-CONCEPTS">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.1. Trigram (or Trigraph) Concepts <a class="id_link" href="#PGTRGM-CONCEPTS">#</a></h3>
</div>
</div>
</div>
<p>A trigram is a group of three consecutive characters taken from a string. We can measure the similarity of two strings by counting the number of trigrams they share. This simple idea turns out to be very effective for measuring the similarity of words in many natural languages.</p>
<div class="note">
<h3 class="title">Note</h3>
<p><code class="filename">pg_trgm</code> ignores non-word characters (non-alphanumerics) when extracting trigrams from a string. Each word is considered to have two spaces prefixed and one space suffixed when determining the set of trigrams contained in the string. For example, the set of trigrams in the string <span class="quote">“<span class="quote"><code class="literal">cat</code></span>”</span> is <span class="quote">“ <span class="quote"><code class="literal">c</code></span>”</span>, <span class="quote">“ <span class="quote"><code class="literal">ca</code></span>”</span>, <span class="quote">“<span class="quote"><code class="literal">cat</code></span>”</span>, and <span class="quote">“<span class="quote"><code class="literal">at</code></span> ”</span>. The set of trigrams in the string <span class="quote">“<span class="quote"><code class="literal">foo|bar</code></span>”</span> is <span class="quote">“ <span class="quote"><code class="literal">f</code></span>”</span>, <span class="quote">“ <span class="quote"><code class="literal">fo</code></span>”</span>, <span class="quote">“<span class="quote"><code class="literal">foo</code></span>”</span>, <span class="quote">“<span class="quote"><code class="literal">oo</code></span> ”</span>, <span class="quote">“ <span class="quote"><code class="literal">b</code></span>”</span>, <span class="quote">“ <span class="quote"><code class="literal">ba</code></span>”</span>, <span class="quote">“<span class="quote"><code class="literal">bar</code></span>”</span>, and <span class="quote">“<span class="quote"><code class="literal">ar</code></span> ”</span>.</p>
</div>
</div>
<div class="sect2" id="PGTRGM-FUNCS-OPS">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.2. Functions and Operators <a class="id_link" href="#PGTRGM-FUNCS-OPS">#</a></h3>
</div>
</div>
</div>
<p>The functions provided by the <code class="filename">pg_trgm</code> module are shown in <a class="xref" href="/docs/current/pgtrgm.html#PGTRGM-FUNC-TABLE" title="Table F.26. pg_trgm Functions">Table F.26</a>, the operators in <a class="xref" href="/docs/current/pgtrgm.html#PGTRGM-OP-TABLE" title="Table F.27. pg_trgm Operators">Table F.27</a>.</p>
<div class="table" id="PGTRGM-FUNC-TABLE">
<p class="title"><strong>Table F.26. <code class="filename">pg_trgm</code> Functions</strong></p>
<div class="table-contents">
<table border="1" class="table" summary="pg_trgm Functions">
<colgroup>
<col/>
</colgroup>
<thead>
<tr>
<th class="func_table_entry">
<p class="func_signature">Function</p>
<p>Description</p>
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.1.1.1.1" name="id-1.11.7.45.6.3.2.2.1.1.1.1"></a> <code class="function">similarity</code> ( <code class="type">text</code>, <code class="type">text</code> ) → <code class="returnvalue">real</code></p>
<p>Returns a number that indicates how similar the two arguments are. The range of the result is zero (indicating that the two strings are completely dissimilar) to one (indicating that the two strings are identical).</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.2.1.1.1" name="id-1.11.7.45.6.3.2.2.2.1.1.1"></a> <code class="function">show_trgm</code> ( <code class="type">text</code> ) → <code class="returnvalue">text[]</code></p>
<p>Returns an array of all the trigrams in the given string. (In practice this is seldom useful except for debugging.)</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.3.1.1.1" name="id-1.11.7.45.6.3.2.2.3.1.1.1"></a> <code class="function">word_similarity</code> ( <code class="type">text</code>, <code class="type">text</code> ) → <code class="returnvalue">real</code></p>
<p>Returns a number that indicates the greatest similarity between the set of trigrams in the first string and any continuous extent of an ordered set of trigrams in the second string. For details, see the explanation below.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.4.1.1.1" name="id-1.11.7.45.6.3.2.2.4.1.1.1"></a> <code class="function">strict_word_similarity</code> ( <code class="type">text</code>, <code class="type">text</code> ) → <code class="returnvalue">real</code></p>
<p>Same as <code class="function">word_similarity</code>, but forces extent boundaries to match word boundaries. Since we don't have cross-word trigrams, this function actually returns greatest similarity between first string and any continuous extent of words of the second string.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.5.1.1.1" name="id-1.11.7.45.6.3.2.2.5.1.1.1"></a> <code class="function">show_limit</code> () → <code class="returnvalue">real</code></p>
<p>Returns the current similarity threshold used by the <code class="literal">%</code> operator. This sets the minimum similarity between two words for them to be considered similar enough to be misspellings of each other, for example. (<span class="emphasis"><em>Deprecated</em></span>; instead use <code class="command">SHOW</code> <code class="varname">pg_trgm.similarity_threshold</code>.)</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><a class="indexterm" id="id-1.11.7.45.6.3.2.2.6.1.1.1" name="id-1.11.7.45.6.3.2.2.6.1.1.1"></a> <code class="function">set_limit</code> ( <code class="type">real</code> ) → <code class="returnvalue">real</code></p>
<p>Sets the current similarity threshold that is used by the <code class="literal">%</code> operator. The threshold must be between 0 and 1 (default is 0.3). Returns the same value passed in. (<span class="emphasis"><em>Deprecated</em></span>; instead use <code class="command">SET</code> <code class="varname">pg_trgm.similarity_threshold</code>.)</p>
</td>
</tr>
</tbody>
</table>
</div>
</div><br class="table-break"/>
<p>Consider the following example:</p>
<pre class="programlisting"># SELECT word_similarity('word', 'two words');
word_similarity
-----------------
0.8
(1 row)
</pre>
<p>In the first string, the set of trigrams is <code class="literal">{" w"," wo","wor","ord","rd "}</code>. In the second string, the ordered set of trigrams is <code class="literal">{" t"," tw","two","wo "," w"," wo","wor","ord","rds","ds "}</code>. The most similar extent of an ordered set of trigrams in the second string is <code class="literal">{" w"," wo","wor","ord"}</code>, and the similarity is <code class="literal">0.8</code>.</p>
<p>This function returns a value that can be approximately understood as the greatest similarity between the first string and any substring of the second string. However, this function does not add padding to the boundaries of the extent. Thus, the number of additional characters present in the second string is not considered, except for the mismatched word boundaries.</p>
<p>At the same time, <code class="function">strict_word_similarity</code> selects an extent of words in the second string. In the example above, <code class="function">strict_word_similarity</code> would select the extent of a single word <code class="literal">'words'</code>, whose set of trigrams is <code class="literal">{" w"," wo","wor","ord","rds","ds "}</code>.</p>
<pre class="programlisting"># SELECT strict_word_similarity('word', 'two words'), similarity('word', 'words');
strict_word_similarity | similarity
------------------------+------------
0.571429 | 0.571429
(1 row)
</pre>
<p>Thus, the <code class="function">strict_word_similarity</code> function is useful for finding the similarity to whole words, while <code class="function">word_similarity</code> is more suitable for finding the similarity for parts of words.</p>
<div class="table" id="PGTRGM-OP-TABLE">
<p class="title"><strong>Table F.27. <code class="filename">pg_trgm</code> Operators</strong></p>
<div class="table-contents">
<table border="1" class="table" summary="pg_trgm Operators">
<colgroup>
<col/>
</colgroup>
<thead>
<tr>
<th class="func_table_entry">
<p class="func_signature">Operator</p>
<p>Description</p>
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal">%</code> <code class="type">text</code> → <code class="returnvalue">boolean</code></p>
<p>Returns <code class="literal">true</code> if its arguments have a similarity that is greater than the current similarity threshold set by <code class="varname">pg_trgm.similarity_threshold</code>.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><%</code> <code class="type">text</code> → <code class="returnvalue">boolean</code></p>
<p>Returns <code class="literal">true</code> if the similarity between the trigram set in the first argument and a continuous extent of an ordered trigram set in the second argument is greater than the current word similarity threshold set by <code class="varname">pg_trgm.word_similarity_threshold</code> parameter.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal">%></code> <code class="type">text</code> → <code class="returnvalue">boolean</code></p>
<p>Commutator of the <code class="literal"><%</code> operator.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><<%</code> <code class="type">text</code> → <code class="returnvalue">boolean</code></p>
<p>Returns <code class="literal">true</code> if its second argument has a continuous extent of an ordered trigram set that matches word boundaries, and its similarity to the trigram set of the first argument is greater than the current strict word similarity threshold set by the <code class="varname">pg_trgm.strict_word_similarity_threshold</code> parameter.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal">%>></code> <code class="type">text</code> → <code class="returnvalue">boolean</code></p>
<p>Commutator of the <code class="literal"><<%</code> operator.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><-></code> <code class="type">text</code> → <code class="returnvalue">real</code></p>
<p>Returns the <span class="quote">“<span class="quote">distance</span>”</span> between the arguments, that is one minus the <code class="function">similarity()</code> value.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><<-></code> <code class="type">text</code> → <code class="returnvalue">real</code></p>
<p>Returns the <span class="quote">“<span class="quote">distance</span>”</span> between the arguments, that is one minus the <code class="function">word_similarity()</code> value.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><->></code> <code class="type">text</code> → <code class="returnvalue">real</code></p>
<p>Commutator of the <code class="literal"><<-></code> operator.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><<<-></code> <code class="type">text</code> → <code class="returnvalue">real</code></p>
<p>Returns the <span class="quote">“<span class="quote">distance</span>”</span> between the arguments, that is one minus the <code class="function">strict_word_similarity()</code> value.</p>
</td>
</tr>
<tr>
<td class="func_table_entry">
<p class="func_signature"><code class="type">text</code> <code class="literal"><->>></code> <code class="type">text</code> → <code class="returnvalue">real</code></p>
<p>Commutator of the <code class="literal"><<<-></code> operator.</p>
</td>
</tr>
</tbody>
</table>
</div>
</div><br class="table-break"/>
</div>
<div class="sect2" id="PGTRGM-GUC">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.3. GUC Parameters <a class="id_link" href="#PGTRGM-GUC">#</a></h3>
</div>
</div>
</div>
<div class="variablelist">
<dl class="variablelist">
<dt id="GUC-PGTRGM-SIMILARITY-THRESHOLD"><span class="term"><code class="varname">pg_trgm.similarity_threshold</code> (<code class="type">real</code>) <a class="indexterm" id="id-1.11.7.45.7.2.1.1.3" name="id-1.11.7.45.7.2.1.1.3"></a></span> <a class="id_link" href="#GUC-PGTRGM-SIMILARITY-THRESHOLD">#</a></dt>
<dd>
<p>Sets the current similarity threshold that is used by the <code class="literal">%</code> operator. The threshold must be between 0 and 1 (default is 0.3).</p>
</dd>
<dt id="GUC-PGTRGM-WORD-SIMILARITY-THRESHOLD"><span class="term"><code class="varname">pg_trgm.word_similarity_threshold</code> (<code class="type">real</code>) <a class="indexterm" id="id-1.11.7.45.7.2.2.1.3" name="id-1.11.7.45.7.2.2.1.3"></a></span> <a class="id_link" href="#GUC-PGTRGM-WORD-SIMILARITY-THRESHOLD">#</a></dt>
<dd>
<p>Sets the current word similarity threshold that is used by the <code class="literal"><%</code> and <code class="literal">%></code> operators. The threshold must be between 0 and 1 (default is 0.6).</p>
</dd>
<dt id="GUC-PGTRGM-STRICT-WORD-SIMILARITY-THRESHOLD"><span class="term"><code class="varname">pg_trgm.strict_word_similarity_threshold</code> (<code class="type">real</code>) <a class="indexterm" id="id-1.11.7.45.7.2.3.1.3" name="id-1.11.7.45.7.2.3.1.3"></a></span> <a class="id_link" href="#GUC-PGTRGM-STRICT-WORD-SIMILARITY-THRESHOLD">#</a></dt>
<dd>
<p>Sets the current strict word similarity threshold that is used by the <code class="literal"><<%</code> and <code class="literal">%>></code> operators. The threshold must be between 0 and 1 (default is 0.5).</p>
</dd>
</dl>
</div>
</div>
<div class="sect2" id="PGTRGM-INDEX">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.4. Index Support <a class="id_link" href="#PGTRGM-INDEX">#</a></h3>
</div>
</div>
</div>
<p>The <code class="filename">pg_trgm</code> module provides GiST and GIN index operator classes that allow you to create an index over a text column for the purpose of very fast similarity searches. These index types support the above-described similarity operators, and additionally support trigram-based index searches for <code class="literal">LIKE</code>, <code class="literal">ILIKE</code>, <code class="literal">~</code>, <code class="literal">~*</code> and <code class="literal">=</code> queries. The similarity comparisons are case-insensitive in a default build of <code class="filename">pg_trgm</code>. Inequality operators are not supported. Note that those indexes may not be as efficient as regular B-tree indexes for equality operator.</p>
<p>Example:</p>
<pre class="programlisting">CREATE TABLE test_trgm (t text);
CREATE INDEX trgm_idx ON test_trgm USING GIST (t gist_trgm_ops);
</pre>
<p>or</p>
<pre class="programlisting">CREATE INDEX trgm_idx ON test_trgm USING GIN (t gin_trgm_ops);
</pre>
<p><code class="literal">gist_trgm_ops</code> GiST opclass approximates a set of trigrams as a bitmap signature. Its optional integer parameter <code class="literal">siglen</code> determines the signature length in bytes. The default length is 12 bytes. Valid values of signature length are between 1 and 2024 bytes. Longer signatures lead to a more precise search (scanning a smaller fraction of the index and fewer heap pages), at the cost of a larger index.</p>
<p>Example of creating such an index with a signature length of 32 bytes:</p>
<pre class="programlisting">CREATE INDEX trgm_idx ON test_trgm USING GIST (t gist_trgm_ops(siglen=32));
</pre>
<p>At this point, you will have an index on the <code class="structfield">t</code> column that you can use for similarity searching. A typical query is</p>
<pre class="programlisting">SELECT t, similarity(t, '<em class="replaceable"><code>word</code></em>') AS sml
FROM test_trgm
WHERE t % '<em class="replaceable"><code>word</code></em>'
ORDER BY sml DESC, t;
</pre>
<p>This will return all values in the text column that are sufficiently similar to <em class="replaceable"><code>word</code></em>, sorted from best match to worst. The index will be used to make this a fast operation even over very large data sets.</p>
<p>A variant of the above query is</p>
<pre class="programlisting">SELECT t, t <-> '<em class="replaceable"><code>word</code></em>' AS dist
FROM test_trgm
ORDER BY dist LIMIT 10;
</pre>
<p>This can be implemented quite efficiently by GiST indexes, but not by GIN indexes. It will usually beat the first formulation when only a small number of the closest matches is wanted.</p>
<p>Also you can use an index on the <code class="structfield">t</code> column for word similarity or strict word similarity. Typical queries are:</p>
<pre class="programlisting">SELECT t, word_similarity('<em class="replaceable"><code>word</code></em>', t) AS sml
FROM test_trgm
WHERE '<em class="replaceable"><code>word</code></em>' <% t
ORDER BY sml DESC, t;
</pre>
<p>and</p>
<pre class="programlisting">SELECT t, strict_word_similarity('<em class="replaceable"><code>word</code></em>', t) AS sml
FROM test_trgm
WHERE '<em class="replaceable"><code>word</code></em>' <<% t
ORDER BY sml DESC, t;
</pre>
<p>This will return all values in the text column for which there is a continuous extent in the corresponding ordered trigram set that is sufficiently similar to the trigram set of <em class="replaceable"><code>word</code></em>, sorted from best match to worst. The index will be used to make this a fast operation even over very large data sets.</p>
<p>Possible variants of the above queries are:</p>
<pre class="programlisting">SELECT t, '<em class="replaceable"><code>word</code></em>' <<-> t AS dist
FROM test_trgm
ORDER BY dist LIMIT 10;
</pre>
<p>and</p>
<pre class="programlisting">SELECT t, '<em class="replaceable"><code>word</code></em>' <<<-> t AS dist
FROM test_trgm
ORDER BY dist LIMIT 10;
</pre>
<p>This can be implemented quite efficiently by GiST indexes, but not by GIN indexes.</p>
<p>Beginning in <span class="productname">PostgreSQL</span> 9.1, these index types also support index searches for <code class="literal">LIKE</code> and <code class="literal">ILIKE</code>, for example</p>
<pre class="programlisting">SELECT * FROM test_trgm WHERE t LIKE '%foo%bar';
</pre>
<p>The index search works by extracting trigrams from the search string and then looking these up in the index. The more trigrams in the search string, the more effective the index search is. Unlike B-tree based searches, the search string need not be left-anchored.</p>
<p>Beginning in <span class="productname">PostgreSQL</span> 9.3, these index types also support index searches for regular-expression matches (<code class="literal">~</code> and <code class="literal">~*</code> operators), for example</p>
<pre class="programlisting">SELECT * FROM test_trgm WHERE t ~ '(foo|bar)';
</pre>
<p>The index search works by extracting trigrams from the regular expression and then looking these up in the index. The more trigrams that can be extracted from the regular expression, the more effective the index search is. Unlike B-tree based searches, the search string need not be left-anchored.</p>
<p>For both <code class="literal">LIKE</code> and regular-expression searches, keep in mind that a pattern with no extractable trigrams will degenerate to a full-index scan.</p>
<p>The choice between GiST and GIN indexing depends on the relative performance characteristics of GiST and GIN, which are discussed elsewhere.</p>
</div>
<div class="sect2" id="PGTRGM-TEXT-SEARCH">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.5. Text Search Integration <a class="id_link" href="#PGTRGM-TEXT-SEARCH">#</a></h3>
</div>
</div>
</div>
<p>Trigram matching is a very useful tool when used in conjunction with a full text index. In particular it can help to recognize misspelled input words that will not be matched directly by the full text search mechanism.</p>
<p>The first step is to generate an auxiliary table containing all the unique words in the documents:</p>
<pre class="programlisting">CREATE TABLE words AS SELECT word FROM
ts_stat('SELECT to_tsvector(''simple'', bodytext) FROM documents');
</pre>
<p>where <code class="structname">documents</code> is a table that has a text field <code class="structfield">bodytext</code> that we wish to search. The reason for using the <code class="literal">simple</code> configuration with the <code class="function">to_tsvector</code> function, instead of using a language-specific configuration, is that we want a list of the original (unstemmed) words.</p>
<p>Next, create a trigram index on the word column:</p>
<pre class="programlisting">CREATE INDEX words_idx ON words USING GIN (word gin_trgm_ops);
</pre>
<p>Now, a <code class="command">SELECT</code> query similar to the previous example can be used to suggest spellings for misspelled words in user search terms. A useful extra test is to require that the selected words are also of similar length to the misspelled word.</p>
<div class="note">
<h3 class="title">Note</h3>
<p>Since the <code class="structname">words</code> table has been generated as a separate, static table, it will need to be periodically regenerated so that it remains reasonably up-to-date with the document collection. Keeping it exactly current is usually unnecessary.</p>
</div>
</div>
<div class="sect2" id="PGTRGM-REFERENCES">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.6. References <a class="id_link" href="#PGTRGM-REFERENCES">#</a></h3>
</div>
</div>
</div>
<p>GiST Development Site <a class="ulink" href="/_404.html" target="_top">http://www.sai.msu.su/~megera/postgres/gist/</a></p>
<p>Tsearch2 Development Site <a class="ulink" href="/_404.html" target="_top">http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/</a></p>
</div>
<div class="sect2" id="PGTRGM-AUTHORS">
<div class="titlepage">
<div>
<div>
<h3 class="title">F.35.7. Authors <a class="id_link" href="#PGTRGM-AUTHORS">#</a></h3>
</div>
</div>
</div>
<p>Oleg Bartunov <code class="email"><<a class="email" href="mailto:oleg@sai.msu.su">oleg@sai.msu.su</a>></code>, Moscow, Moscow University, Russia</p>
<p>Teodor Sigaev <code class="email"><<a class="email" href="mailto:teodor@sigaev.ru">teodor@sigaev.ru</a>></code>, Moscow, Delta-Soft Ltd.,Russia</p>
<p>Alexander Korotkov <code class="email"><<a class="email" href="mailto:a.korotkov@postgrespro.ru">a.korotkov@postgrespro.ru</a>></code>, Moscow, Postgres Professional, Russia</p>
<p>Documentation: Christopher Kings-Lynne</p>
<p>This module is sponsored by Delta-Soft Ltd., Moscow, Russia.</p>
</div>
</div>
<div class="navfooter">
<hr/>
<table summary="Navigation footer" width="100%">
<tbody><tr>
<td align="left" width="40%"><a accesskey="p" href="/docs/current/pgsurgery.html" title="F.34. pg_surgery — perform low-level surgery on relation data">Prev</a> </td>
<td align="center" width="20%"><a accesskey="u" href="/docs/current/contrib.html" title="Appendix F. Additional Supplied Modules and Extensions">Up</a></td>
<td align="right" width="40%"> <a accesskey="n" href="/docs/current/pgvisibility.html" title="F.36. pg_visibility — visibility map information and utilities">Next</a></td>
</tr>
<tr>
<td align="left" valign="top" width="40%">F.34. pg_surgery — perform low-level surgery on relation data </td>
<td align="center" width="20%"><a accesskey="h" href="/docs/current/index.html" title="PostgreSQL 18.3 Documentation">Home</a></td>
<td align="right" valign="top" width="40%"> F.36. pg_visibility — visibility map information and utilities</td>
</tr>
</tbody></table>
</div>
</div>
<div id="docComments">
<h2>Submit correction</h2>
<p>
If you see anything in the documentation that is not correct, does not match
your experience with the particular feature or requires further clarification,
please use
<a href="/account/comments/new/18/pgtrgm.html/" rel="nofollow">this form</a>
to report a documentation issue.
</p>
</div>
</div> <!-- pgContentWrap -->
</div>
</div>
<!-- Footer -->
<footer id="footer">
<!-- Copyright -->
<div class="container">
<a href="/about/policies/">Policies</a> |
<a href="/about/policies/coc/">Code of Conduct</a> |
<a href="/about/">About PostgreSQL</a> |
<a href="/about/contact/">Contact</a><br/>
<p>Copyright © 1996-2026 The PostgreSQL Global Development Group</p>
</div>
</footer>
<script src="/media/js/main.js?54a05eb2"></script>
</body></html> |