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authorBjörn Gustavsson <[email protected]>2015-03-12 15:35:13 +0100
committerBjörn Gustavsson <[email protected]>2015-03-12 15:38:25 +0100
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Update Efficiency Guide
Language cleaned up by the technical writers xsipewe and tmanevik from Combitech. Proofreading and corrections by Björn Gustavsson.
Diffstat (limited to 'system/doc/efficiency_guide/tablesDatabases.xml')
-rw-r--r--system/doc/efficiency_guide/tablesDatabases.xml194
1 files changed, 98 insertions, 96 deletions
diff --git a/system/doc/efficiency_guide/tablesDatabases.xml b/system/doc/efficiency_guide/tablesDatabases.xml
index 94c921fa1c..215c2afa1f 100644
--- a/system/doc/efficiency_guide/tablesDatabases.xml
+++ b/system/doc/efficiency_guide/tablesDatabases.xml
@@ -18,10 +18,9 @@
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights 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 +29,45 @@
</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 +86,16 @@ 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 +136,7 @@ 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 +151,31 @@ 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 +192,8 @@ 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 +224,8 @@ 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 +243,60 @@ 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 +306,31 @@ 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 +348,15 @@ 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