1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Access Methods</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="introduction.html" title="Chapter��1.��Introduction to Berkeley DB" /> 11 <link rel="prev" href="javadplconcepts.html" title="Berkeley DB Concepts" /> 12 <link rel="next" href="databaseLimits.html" title="Database Limits and Portability" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Access Methods</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="javadplconcepts.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��1.��Introduction to Berkeley DB </th> 23 <td width="20%" align="right">��<a accesskey="n" href="databaseLimits.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="accessmethods"></a>Access Methods</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <dl> 38 <dt> 39 <span class="sect2"> 40 <a href="accessmethods.html#selectAM">Selecting Access Methods</a> 41 </span> 42 </dt> 43 <dt> 44 <span class="sect2"> 45 <a href="accessmethods.html#BTreeVSHash">Choosing between BTree and Hash</a> 46 </span> 47 </dt> 48 <dt> 49 <span class="sect2"> 50 <a href="accessmethods.html#QueueVSRecno">Choosing between Queue and Recno</a> 51 </span> 52 </dt> 53 </dl> 54 </div> 55 <p> 56 While this manual will focus primarily on the BTree access method, it is 57 still useful to briefly describe all of the access methods that DB 58 makes available. 59 </p> 60 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 61 <h3 class="title">Note</h3> 62 <p> 63 If you are using the DPL, be aware that it only 64 supports the BTree access method. For that reason, you 65 can skip this section. 66 </p> 67 </div> 68 <p> 69 Note that an access method can be selected only when the database is 70 created. Once selected, actual API usage is generally 71 identical across all access methods. That is, while some 72 exceptions exist, mechanically you interact with the library in the same 73 way regardless of which access method you have selected. 74 </p> 75 <p> 76 The access method that you should choose is gated first by what you want 77 to use as a key, and then secondly by the performance that you see 78 for a given access method. 79 </p> 80 <p> 81 The following are the available access methods: 82 </p> 83 <div class="informaltable"> 84 <table border="1" width="80%"> 85 <colgroup> 86 <col align="left" /> 87 <col align="left" /> 88 </colgroup> 89 <thead> 90 <tr> 91 <th align="center">Access Method</th> 92 <th align="center">Description</th> 93 </tr> 94 </thead> 95 <tbody> 96 <tr> 97 <td align="left">BTree</td> 98 <td align="left" valign="top"> 99 <p> 100 Data is stored in a sorted, balanced tree structure. 101 Both the key and the data for BTree records can be 102 arbitrarily complex. That is, they can contain single values 103 such as an integer or a string, or complex types such as a 104 structure. Also, although not the default 105 behavior, it is possible for two records to 106 use keys that compare as equals. When this occurs, the 107 records are considered to be duplicates of one another. 108 </p> 109 </td> 110 </tr> 111 <tr> 112 <td align="left">Hash</td> 113 <td align="left" valign="top"> 114 <p> 115 Data is stored in an extended linear hash table. Like 116 BTree, the key and the data used for Hash records can be of 117 arbitrarily complex data. Also, like BTree, duplicate 118 records are optionally supported. 119 </p> 120 </td> 121 </tr> 122 <tr> 123 <td align="left">Queue</td> 124 <td align="left" valign="top"> 125 <p> 126 Data is stored in a queue as fixed-length records. Each 127 record uses a logical record number as its key. This access 128 method is designed for fast inserts at the tail of the 129 queue, and it has a special operation that deletes and 130 returns a record from the head of the queue. 131 </p> 132 <p> 133 This access method is unusual in that it provides record 134 level locking. This can provide 135 beneficial performance improvements in applications 136 requiring concurrent access to the queue. 137 </p> 138 </td> 139 </tr> 140 <tr> 141 <td align="left">Recno</td> 142 <td align="left" valign="top"> 143 <p> 144 Data is stored in either fixed or variable-length records. 145 Like Queue, Recno records use logical record numbers as keys. 146 </p> 147 </td> 148 </tr> 149 </tbody> 150 </table> 151 </div> 152 <div class="sect2" lang="en" xml:lang="en"> 153 <div class="titlepage"> 154 <div> 155 <div> 156 <h3 class="title"><a id="selectAM"></a>Selecting Access Methods</h3> 157 </div> 158 </div> 159 </div> 160 <p> 161 To select an access method, you should first consider what you want 162 to use as a key for you database records. If you want to use 163 arbitrary data (even strings), then you should use either BTree or 164 Hash. If you want to use logical record numbers (essentially 165 integers) then you should use Queue or Recno. 166 </p> 167 <p> 168 Once you have made this decision, you must choose between either 169 BTree or Hash, or Queue or Recno. This decision is described next. 170 </p> 171 </div> 172 <div class="sect2" lang="en" xml:lang="en"> 173 <div class="titlepage"> 174 <div> 175 <div> 176 <h3 class="title"><a id="BTreeVSHash"></a>Choosing between BTree and Hash</h3> 177 </div> 178 </div> 179 </div> 180 <p> 181 For small working datasets that fit entirely in memory, there is no 182 difference between BTree and Hash. Both will perform just as well 183 as the other. In this situation, you might just as well use BTree, 184 if for no other reason than the majority of DB applications use 185 BTree. 186 </p> 187 <p> 188 Note that the main concern here is your 189 working dataset, not your entire dataset. Many applications maintain 190 large amounts of information but only need to access some small 191 portion of that data with any frequency. So what you want to 192 consider is the data that you will routinely use, not the sum total 193 of all the data managed by your application. 194 </p> 195 <p> 196 However, as your working dataset grows to the point 197 where you cannot fit it all into memory, then you need to take more 198 care when choosing your access method. Specifically, choose: 199 </p> 200 <div class="itemizedlist"> 201 <ul type="disc"> 202 <li> 203 <p> 204 BTree if your keys have some locality of reference. That is, 205 if they sort well and you can expect that a query for a 206 given key will likely be followed by a query for one of its 207 neighbors. 208 </p> 209 </li> 210 <li> 211 <p> 212 Hash if your dataset is extremely large. For any given 213 access method, DB must maintain a certain amount of internal 214 information. However, the amount of information that DB 215 must maintain for BTree is much greater than for Hash. The 216 result is that as your dataset grows, this internal 217 information can dominate the cache to the point where there 218 is relatively little space left for application data. 219 As a result, BTree can be forced to perform disk I/O much more 220 frequently than would Hash given the same amount of data. 221 </p> 222 <p> 223 Moreover, if your dataset becomes so large that DB will 224 almost certainly have to perform disk I/O to satisfy a 225 random request, then Hash will definitely out perform BTree 226 because it has fewer internal records to search through than 227 does BTree. 228 </p> 229 </li> 230 </ul> 231 </div> 232 </div> 233 <div class="sect2" lang="en" xml:lang="en"> 234 <div class="titlepage"> 235 <div> 236 <div> 237 <h3 class="title"><a id="QueueVSRecno"></a>Choosing between Queue and Recno</h3> 238 </div> 239 </div> 240 </div> 241 <p> 242 Queue or Recno are used when the application wants to use logical 243 record numbers for the primary database key. Logical record numbers 244 are essentially integers that uniquely identify the database 245 record. They can be either mutable or fixed, where a mutable record 246 number is one that might change as database records are stored or 247 deleted. Fixed logical record numbers never change regardless of 248 what database operations are performed. 249 </p> 250 <p> 251 When deciding between Queue and Recno, choose: 252 </p> 253 <div class="itemizedlist"> 254 <ul type="disc"> 255 <li> 256 <p> 257 Queue if your application requires high degrees of 258 concurrency. Queue provides record-level locking (as opposed 259 to the page-level locking that the other access methods 260 use), and this can result in significantly faster throughput 261 for highly concurrent applications. 262 </p> 263 <p> 264 Note, however, that Queue provides support only for fixed 265 length records. So if the size of the data that you want to 266 store varies widely from record to record, you should 267 probably choose an access method other than Queue. 268 </p> 269 </li> 270 <li> 271 <p> 272 Recno if you want mutable record numbers. Queue is only 273 capable of providing fixed record numbers. Also, Recno 274 provides support for databases whose permanent storage is a 275 flat text file. This is useful for applications looking for 276 fast, temporary storage while the data is being read or 277 modified. 278 </p> 279 </li> 280 </ul> 281 </div> 282 </div> 283 </div> 284 <div class="navfooter"> 285 <hr /> 286 <table width="100%" summary="Navigation footer"> 287 <tr> 288 <td width="40%" align="left"><a accesskey="p" href="javadplconcepts.html">Prev</a>��</td> 289 <td width="20%" align="center"> 290 <a accesskey="u" href="introduction.html">Up</a> 291 </td> 292 <td width="40%" align="right">��<a accesskey="n" href="databaseLimits.html">Next</a></td> 293 </tr> 294 <tr> 295 <td width="40%" align="left" valign="top">Berkeley DB Concepts��</td> 296 <td width="20%" align="center"> 297 <a accesskey="h" href="index.html">Home</a> 298 </td> 299 <td width="40%" align="right" valign="top">��Database Limits and Portability</td> 300 </tr> 301 </table> 302 </div> 303 </body> 304</html> 305