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This allows these modules to be installed into a database without superuser privileges (assuming that the DBA or sysadmin has installed the module's files in the expected place). You only need CREATE privilege on the current database, which by default would be available to the database owner. The following modules are marked trusted: btree_gin btree_gist citext cube dict_int earthdistance fuzzystrmatch hstore hstore_plperl intarray isn jsonb_plperl lo ltree pg_trgm pgcrypto seg tablefunc tcn tsm_system_rows tsm_system_time unaccent uuid-ossp In the future we might mark some more modules trusted, but there seems to be no debate about these, and on the whole it seems wise to be conservative with use of this feature to start out with. Discussion: https://postgr.es/m/32315.1580326876@sss.pgh.pa.us
72 lines
2.1 KiB
Plaintext
72 lines
2.1 KiB
Plaintext
<!-- doc/src/sgml/tsm-system-time.sgml -->
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<sect1 id="tsm-system-time" xreflabel="tsm_system_time">
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<title>tsm_system_time</title>
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<indexterm zone="tsm-system-time">
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<primary>tsm_system_time</primary>
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</indexterm>
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<para>
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The <filename>tsm_system_time</filename> module provides the table sampling method
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<literal>SYSTEM_TIME</literal>, which can be used in
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the <literal>TABLESAMPLE</literal> clause of a <xref linkend="sql-select"/>
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command.
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</para>
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<para>
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This table sampling method accepts a single floating-point argument that
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is the maximum number of milliseconds to spend reading the table. This
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gives you direct control over how long the query takes, at the price that
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the size of the sample becomes hard to predict. The resulting sample will
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contain as many rows as could be read in the specified time, unless the
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whole table has been read first.
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</para>
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<para>
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Like the built-in <literal>SYSTEM</literal> sampling
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method, <literal>SYSTEM_TIME</literal> performs block-level sampling, so
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that the sample is not completely random but may be subject to clustering
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effects, especially if only a small number of rows are selected.
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</para>
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<para>
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<literal>SYSTEM_TIME</literal> does not support
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the <literal>REPEATABLE</literal> clause.
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</para>
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<para>
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This module is considered <quote>trusted</quote>, that is, it can be
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installed by non-superusers who have <literal>CREATE</literal> privilege
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on the current database.
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</para>
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<sect2>
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<title>Examples</title>
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<para>
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Here is an example of selecting a sample of a table with
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<literal>SYSTEM_TIME</literal>. First install the extension:
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</para>
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<programlisting>
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CREATE EXTENSION tsm_system_time;
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</programlisting>
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<para>
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Then you can use it in a <command>SELECT</command> command, for instance:
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<programlisting>
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SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
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</programlisting>
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</para>
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<para>
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This command will return as large a sample of <structname>my_table</structname> as
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it can read in 1 second (1000 milliseconds). Of course, if the whole
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table can be read in under 1 second, all its rows will be returned.
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</para>
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</sect2>
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</sect1>
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