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diff --git a/system/doc/efficiency_guide/tablesDatabases.xml b/system/doc/efficiency_guide/tablesDatabases.xml index 94c921fa1c..865c46bc33 100644 --- a/system/doc/efficiency_guide/tablesDatabases.xml +++ b/system/doc/efficiency_guide/tablesDatabases.xml @@ -8,20 +8,20 @@ <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> - The contents of this file are subject to the Erlang Public License, - Version 1.1, (the "License"); you may not use this file except in - compliance with the License. You should have received a copy of the - Erlang Public License along with this software. If not, it can be - retrieved online at http://www.erlang.org/. - - Software distributed under the License is distributed on an "AS IS" - basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See - the License for the specific language governing rights and limitations - under the License. - + Licensed under the Apache License, Version 2.0 (the "License"); + you may not use this file except in compliance with the License. + You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. </legalnotice> - <title>Tables and databases</title> + <title>Tables and Databases</title> <prepared>Ingela Anderton</prepared> <docno></docno> <date>2001-08-07</date> @@ -30,46 +30,45 @@ </header> <section> - <title>Ets, Dets and Mnesia</title> + <title>Ets, Dets, and Mnesia</title> <p>Every example using Ets has a corresponding example in - Mnesia. In general all Ets examples also apply to Dets tables.</p> + Mnesia. In general, all Ets examples also apply to Dets tables.</p> <section> - <title>Select/Match operations</title> - <p>Select/Match operations on Ets and Mnesia tables can become + <title>Select/Match Operations</title> + <p>Select/match operations on Ets and Mnesia tables can become very expensive operations. They usually need to scan the complete - table. You should try to structure your - data so that you minimize the need for select/match - operations. However, if you really need a select/match operation, - it will still be more efficient than using <c>tab2list</c>. - Examples of this and also of ways to avoid select/match will be provided in - some of the following sections. The functions - <c>ets:select/2</c> and <c>mnesia:select/3</c> should be preferred over - <c>ets:match/2</c>,<c>ets:match_object/2</c>, and <c>mnesia:match_object/3</c>.</p> - <note> - <p>There are exceptions when the complete table is not - scanned, for instance if part of the key is bound when searching an - <c>ordered_set</c> table, or if it is a Mnesia - table and there is a secondary index on the field that is - selected/matched. If the key is fully bound there will, of course, be - no point in doing a select/match, unless you have a bag table and - you are only interested in a sub-set of the elements with - the specific key.</p> - </note> - <p>When creating a record to be used in a select/match operation you - want most of the fields to have the value '_'. The easiest and fastest way - to do that is as follows:</p> + table. Try to structure the data to minimize the need for select/match + operations. However, if you require a select/match operation, + it is still more efficient than using <c>tab2list</c>. + Examples of this and of how to avoid select/match are provided in + the following sections. The functions + <c>ets:select/2</c> and <c>mnesia:select/3</c> are to be preferred + over <c>ets:match/2</c>, <c>ets:match_object/2</c>, and + <c>mnesia:match_object/3</c>.</p> + <p>In some circumstances, the select/match operations do not need + to scan the complete table. + For example, if part of the key is bound when searching an + <c>ordered_set</c> table, or if it is a Mnesia + table and there is a secondary index on the field that is + selected/matched. If the key is fully bound, there is + no point in doing a select/match, unless you have a bag table + and are only interested in a subset of the elements with + the specific key.</p> + <p>When creating a record to be used in a select/match operation, you + want most of the fields to have the value "_". The easiest and + fastest way to do that is as follows:</p> <pre> #person{age = 42, _ = '_'}. </pre> </section> <section> - <title>Deleting an element</title> - <p>The delete operation is considered + <title>Deleting an Element</title> + <p>The <c>delete</c> operation is considered successful if the element was not present in the table. Hence all attempts to check that the element is present in the Ets/Mnesia table before deletion are unnecessary. Here follows - an example for Ets tables.</p> + an example for Ets tables:</p> <p><em>DO</em></p> <pre> ... @@ -88,14 +87,16 @@ end, </section> <section> - <title>Data fetching</title> - <p>Do not fetch data that you already have! Consider that you - have a module that handles the abstract data type Person. You - export the interface function <c>print_person/1</c> that uses the internal functions - <c>print_name/1</c>, <c>print_age/1</c>, <c>print_occupation/1</c>.</p> + <title>Fetching Data</title> + <p>Do not fetch data that you already have.</p> + <p>Consider that you have a module that handles the abstract data + type <c>Person</c>. You export the interface function + <c>print_person/1</c>, which uses the internal functions + <c>print_name/1</c>, <c>print_age/1</c>, and + <c>print_occupation/1</c>.</p> <note> - <p>If the functions <c>print_name/1</c> and so on, had been interface - functions the matter comes in to a whole new light, as you + <p>If the function <c>print_name/1</c>, and so on, had been interface + functions, the situation would have been different, as you do not want the user of the interface to know about the internal data representation. </p> </note> @@ -136,7 +137,7 @@ print_person(PersonId) -> io:format("No person with ID = ~p~n", [PersonID]) end. -%%% Internal functions +%%% Internal functionss print_name(PersonID) -> [Person] = ets:lookup(person, PersonId), io:format("No person ~p~n", [Person#person.name]). @@ -151,31 +152,31 @@ print_occupation(PersonID) -> </section> <section> - <title>Non-persistent data storage </title> + <title>Non-Persistent Database Storage</title> <p>For non-persistent database storage, prefer Ets tables over - Mnesia local_content tables. Even the Mnesia <c>dirty_write</c> + Mnesia <c>local_content</c> tables. Even the Mnesia <c>dirty_write</c> operations carry a fixed overhead compared to Ets writes. Mnesia must check if the table is replicated or has indices, this involves at least one Ets lookup for each - <c>dirty_write</c>. Thus, Ets writes will always be faster than + <c>dirty_write</c>. Thus, Ets writes is always faster than Mnesia writes.</p> </section> <section> <title>tab2list</title> - <p>Assume we have an Ets-table, which uses <c>idno</c> as key, - and contains:</p> + <p>Assuming an Ets table that uses <c>idno</c> as key + and contains the following:</p> <pre> [#person{idno = 1, name = "Adam", age = 31, occupation = "mailman"}, #person{idno = 2, name = "Bryan", age = 31, occupation = "cashier"}, #person{idno = 3, name = "Bryan", age = 35, occupation = "banker"}, #person{idno = 4, name = "Carl", age = 25, occupation = "mailman"}]</pre> - <p>If we <em>must</em> return all data stored in the Ets-table we - can use <c>ets:tab2list/1</c>. However, usually we are only + <p>If you <em>must</em> return all data stored in the Ets table, you + can use <c>ets:tab2list/1</c>. However, usually you are only interested in a subset of the information in which case - <c>ets:tab2list/1</c> is expensive. If we only want to extract - one field from each record, e.g., the age of every person, we - should use:</p> + <c>ets:tab2list/1</c> is expensive. If you only want to extract + one field from each record, for example, the age of every person, + then:</p> <p><em>DO</em></p> <pre> ... @@ -192,8 +193,8 @@ ets:select(Tab,[{ #person{idno='_', TabList = ets:tab2list(Tab), lists:map(fun(X) -> X#person.age end, TabList), ...</pre> - <p>If we are only interested in the age of all persons named - Bryan, we should:</p> + <p>If you are only interested in the age of all persons named + "Bryan", then:</p> <p><em>DO</em></p> <pre> ... @@ -224,8 +225,8 @@ BryanList = lists:filter(fun(X) -> X#person.name == "Bryan" end, TabList), lists:map(fun(X) -> X#person.age end, BryanList), ...</pre> - <p>If we need all information stored in the Ets table about - persons named Bryan we should:</p> + <p>If you need all information stored in the Ets table about + persons named "Bryan", then:</p> <p><em>DO</em></p> <pre> ... @@ -243,60 +244,60 @@ lists:filter(fun(X) -> X#person.name == "Bryan" end, TabList), </section> <section> - <title>Ordered_set tables</title> - <p>If the data in the table should be accessed so that the order + <title>Ordered_set Tables</title> + <p>If the data in the table is to be accessed so that the order of the keys in the table is significant, the table type - <c>ordered_set</c> could be used instead of the more usual + <c>ordered_set</c> can be used instead of the more usual <c>set</c> table type. An <c>ordered_set</c> is always - traversed in Erlang term order with regard to the key field - so that return values from functions such as <c>select</c>, + traversed in Erlang term order regarding the key field + so that the return values from functions such as <c>select</c>, <c>match_object</c>, and <c>foldl</c> are ordered by the key values. Traversing an <c>ordered_set</c> with the <c>first</c> and <c>next</c> operations also returns the keys ordered.</p> <note> <p>An <c>ordered_set</c> only guarantees that - objects are processed in <em>key</em> order. Results from functions as - <c>ets:select/2</c> appear in the <em>key</em> order even if + objects are processed in <em>key</em> order. + Results from functions such as + <c>ets:select/2</c> appear in <em>key</em> order even if the key is not included in the result.</p> </note> </section> </section> <section> - <title>Ets specific</title> + <title>Ets-Specific</title> <section> - <title>Utilizing the keys of the Ets table</title> - <p>An Ets table is a single key table (either a hash table or a - tree ordered by the key) and should be used as one. In other + <title>Using Keys of Ets Table</title> + <p>An Ets table is a single-key table (either a hash table or a + tree ordered by the key) and is to be used as one. In other words, use the key to look up things whenever possible. A - lookup by a known key in a set Ets table is constant and for a - ordered_set Ets table it is O(logN). A key lookup is always + lookup by a known key in a <c>set</c> Ets table is constant and for + an <c>ordered_set</c> Ets table it is O(logN). A key lookup is always preferable to a call where the whole table has to be - scanned. In the examples above, the field <c>idno</c> is the + scanned. In the previous examples, the field <c>idno</c> is the key of the table and all lookups where only the name is known - will result in a complete scan of the (possibly large) table + result in a complete scan of the (possibly large) table for a matching result.</p> <p>A simple solution would be to use the <c>name</c> field as the key instead of the <c>idno</c> field, but that would cause - problems if the names were not unique. A more general solution - would be to create a second table with <c>name</c> as key and - <c>idno</c> as data, i.e. to index (invert) the table with regards - to the <c>name</c> field. The second table would of course have to be - kept consistent with the master table. Mnesia could do this - for you, but a home brew index table could be very efficient + problems if the names were not unique. A more general solution would + be to create a second table with <c>name</c> as key and + <c>idno</c> as data, that is, to index (invert) the table regarding + the <c>name</c> field. Clearly, the second table would have to be + kept consistent with the master table. Mnesia can do this + for you, but a home brew index table can be very efficient compared to the overhead involved in using Mnesia.</p> <p>An index table for the table in the previous examples would - have to be a bag (as keys would appear more than once) and could + have to be a bag (as keys would appear more than once) and can have the following contents:</p> <pre> - [#index_entry{name="Adam", idno=1}, #index_entry{name="Bryan", idno=2}, #index_entry{name="Bryan", idno=3}, #index_entry{name="Carl", idno=4}]</pre> - <p>Given this index table a lookup of the <c>age</c> fields for - all persons named "Bryan" could be done like this:</p> + <p>Given this index table, a lookup of the <c>age</c> fields for + all persons named "Bryan" can be done as follows:</p> <pre> ... MatchingIDs = ets:lookup(IndexTable,"Bryan"), @@ -306,30 +307,31 @@ lists:map(fun(#index_entry{idno = ID}) -> end, MatchingIDs), ...</pre> - <p>Note that the code above never uses <c>ets:match/2</c> but - instead utilizes the <c>ets:lookup/2</c> call. The + <p>Notice that this code never uses <c>ets:match/2</c> but + instead uses the <c>ets:lookup/2</c> call. The <c>lists:map/2</c> call is only used to traverse the <c>idno</c>s - matching the name "Bryan" in the table; therefore the number of lookups + matching the name "Bryan" in the table; thus the number of lookups in the master table is minimized.</p> <p>Keeping an index table introduces some overhead when - inserting records in the table, therefore the number of operations - gained from the table has to be weighted against the number of - operations inserting objects in the table. However, note that the gain when - the key can be used to lookup elements is significant.</p> + inserting records in the table. The number of operations gained + from the table must therefore be compared against the number of + operations inserting objects in the table. However, notice that the + gain is significant when the key can be used to lookup elements.</p> </section> </section> <section> - <title>Mnesia specific</title> + <title>Mnesia-Specific</title> <section> - <title>Secondary index</title> + <title>Secondary Index</title> <p>If you frequently do a lookup on a field that is not the - key of the table, you will lose performance using - "mnesia:select/match_object" as this function will traverse the - whole table. You may create a secondary index instead and + key of the table, you lose performance using + "mnesia:select/match_object" as this function traverses the + whole table. You can create a secondary index instead and use "mnesia:index_read" to get faster access, however this - will require more memory. Example:</p> + requires more memory.</p> + <p><em>Example</em></p> <pre> -record(person, {idno, name, age, occupation}). ... @@ -347,14 +349,15 @@ PersonsAge42 = <section> <title>Transactions </title> - <p>Transactions is a way to guarantee that the distributed + <p>Using transactions is a way to guarantee that the distributed Mnesia database remains consistent, even when many different - processes update it in parallel. However if you have - real time requirements it is recommended to use dirty - operations instead of transactions. When using the dirty - operations you lose the consistency guarantee, this is usually + processes update it in parallel. However, if you have + real-time requirements it is recommended to use <c>dirty</c> + operations instead of transactions. When using <c>dirty</c> + operations, you lose the consistency guarantee; this is usually solved by only letting one process update the table. Other - processes have to send update requests to that process.</p> + processes must send update requests to that process.</p> + <p><em>Example</em></p> <pre> ... % Using transaction |