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| author | Lorry Tar Creator <lorry-tar-importer@baserock.org> | 2015-02-17 17:25:57 +0000 |
|---|---|---|
| committer | <> | 2015-03-17 16:26:24 +0000 |
| commit | 780b92ada9afcf1d58085a83a0b9e6bc982203d1 (patch) | |
| tree | 598f8b9fa431b228d29897e798de4ac0c1d3d970 /docs/programmer_reference/hash_conf.html | |
| parent | 7a2660ba9cc2dc03a69ddfcfd95369395cc87444 (diff) | |
| download | berkeleydb-master.tar.gz | |
Diffstat (limited to 'docs/programmer_reference/hash_conf.html')
| -rw-r--r-- | docs/programmer_reference/hash_conf.html | 116 |
1 files changed, 70 insertions, 46 deletions
diff --git a/docs/programmer_reference/hash_conf.html b/docs/programmer_reference/hash_conf.html index 51f31bf2..89879261 100644 --- a/docs/programmer_reference/hash_conf.html +++ b/docs/programmer_reference/hash_conf.html @@ -14,17 +14,16 @@ <body> <div xmlns="" class="navheader"> <div class="libver"> - <p>Library Version 11.2.5.3</p> + <p>Library Version 12.1.6.1</p> </div> <table width="100%" summary="Navigation header"> <tr> - <th colspan="3" align="center">Hash access method specific configuration</th> + <th colspan="3" align="center">Hash access method specific + configuration</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="bt_conf.html">Prev</a> </td> - <th width="60%" align="center">Chapter 2. - Access Method Configuration - </th> + <th width="60%" align="center">Chapter 2. Access Method Configuration </th> <td width="20%" align="right"> <a accesskey="n" href="heap_conf.html">Next</a></td> </tr> </table> @@ -34,7 +33,8 @@ <div class="titlepage"> <div> <div> - <h2 class="title" style="clear: both"><a id="hash_conf"></a>Hash access method specific configuration</h2> + <h2 class="title" style="clear: both"><a id="hash_conf"></a>Hash access method specific + configuration</h2> </div> </div> </div> @@ -57,10 +57,11 @@ </dt> </dl> </div> - <p> - There are a series of configuration tasks which you can perform when - using the Hash access method. They are described in the following sections. -</p> + <p> + There are a series of configuration tasks which you can + perform when using the Hash access method. They are described + in the following sections. + </p> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> <div> @@ -69,15 +70,21 @@ </div> </div> </div> - <p>The density, or page fill factor, is an approximation of the number of -keys allowed to accumulate in any one bucket, determining when the hash -table grows or shrinks. If you know the average sizes of the keys and -data in your data set, setting the fill factor can enhance performance. -A reasonable rule to use to compute fill factor is:</p> + <p> + The density, or page fill factor, is an approximation of the + number of keys allowed to accumulate in any one bucket, + determining when the hash table grows or shrinks. If you know + the average sizes of the keys and data in your data set, + setting the fill factor can enhance performance. A reasonable + rule to use to compute fill factor is: + </p> <pre class="programlisting">(pagesize - 32) / (average_key_size + average_data_size + 8)</pre> - <p>The desired density within the hash table can be specified by calling -the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> method. If no density is specified, one will -be selected dynamically as pages are filled.</p> + <p> + The desired density within the hash table can be specified + by calling the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> method. If no density is + specified, one will be selected dynamically as pages are + filled. + </p> </div> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> @@ -87,23 +94,32 @@ be selected dynamically as pages are filled.</p> </div> </div> </div> - <p>The database hash determines in which bucket a particular key will reside. -The goal of hashing keys is to distribute keys equally across the database -pages, therefore it is important that the hash function work well with -the specified keys so that the resulting bucket usage is relatively -uniform. A hash function that does not work well can effectively turn -into a sequential list.</p> - <p>No hash performs equally well on all possible data sets. It is possible -that applications may find that the default hash function performs poorly -with a particular set of keys. The distribution resulting from the hash -function can be checked using the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility. By comparing the -number of hash buckets and the number of keys, one can decide if the entries -are hashing in a well-distributed manner.</p> - <p>The hash function for the hash table can be specified by calling the -<a href="../api_reference/C/dbset_h_hash.html" class="olink">DB->set_h_hash()</a> method. If no hash function is specified, a default -function will be used. Any application-specified hash function must -take a reference to a <a href="../api_reference/C/db.html" class="olink">DB</a> object, a pointer to a byte string and -its length, as arguments and return an unsigned, 32-bit hash value.</p> + <p> + The database hash determines in which bucket a particular + key will reside. The goal of hashing keys is to distribute + keys equally across the database pages, therefore it is + important that the hash function work well with the specified + keys so that the resulting bucket usage is relatively uniform. + A hash function that does not work well can effectively turn + into a sequential list. + </p> + <p> + No hash performs equally well on all possible data sets. It + is possible that applications may find that the default hash + function performs poorly with a particular set of keys. The + distribution resulting from the hash function can be checked + using the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility. By comparing the number of hash buckets + and the number of keys, one can decide if the entries are + hashing in a well-distributed manner. + </p> + <p> + The hash function for the hash table can be specified by + calling the <a href="../api_reference/C/dbset_h_hash.html" class="olink">DB->set_h_hash()</a> method. If no hash function is + specified, a default function will be used. Any + application-specified hash function must take a reference to a + <a href="../api_reference/C/db.html" class="olink">DB</a> object, a pointer to a byte string and its length, as + arguments and return an unsigned, 32-bit hash value. + </p> </div> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> @@ -113,16 +129,23 @@ its length, as arguments and return an unsigned, 32-bit hash value.</p> </div> </div> </div> - <p>When setting up the hash database, knowing the expected number of elements -that will be stored in the hash table is useful. This value can be used -by the Hash access method implementation to more accurately construct the -necessary number of buckets that the database will eventually require.</p> - <p>The anticipated number of elements in the hash table can be specified by -calling the <a href="../api_reference/C/dbset_h_nelem.html" class="olink">DB->set_h_nelem()</a> method. If not specified, or set too low, -hash tables will expand gracefully as keys are entered, although a slight -performance degradation may be noticed. In order for the estimated number -of elements to be a useful value to Berkeley DB, the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> method -must also be called to set the page fill factor.</p> + <p> + When setting up the hash database, knowing the expected + number of elements that will be stored in the hash table is + useful. This value can be used by the Hash access method + implementation to more accurately construct the necessary + number of buckets that the database will eventually + require. + </p> + <p> + The anticipated number of elements in the hash table can be + specified by calling the <a href="../api_reference/C/dbset_h_nelem.html" class="olink">DB->set_h_nelem()</a> method. If not + specified, or set too low, hash tables will expand gracefully + as keys are entered, although a slight performance degradation + may be noticed. In order for the estimated number of elements + to be a useful value to Berkeley DB, the <a href="../api_reference/C/dbset_h_ffactor.html" class="olink">DB->set_h_ffactor()</a> + method must also be called to set the page fill factor. + </p> </div> </div> <div class="navfooter"> @@ -140,7 +163,8 @@ must also be called to set the page fill factor.</p> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> - <td width="40%" align="right" valign="top"> Heap access method specific configuration</td> + <td width="40%" align="right" valign="top"> Heap access method specific + configuration</td> </tr> </table> </div> |