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>CLUSTER</TITLE
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><H1
><A
NAME="SQL-CLUSTER"
></A
>CLUSTER</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN66547"
></A
><H2
>Name</H2
>CLUSTER -- cluster a table according to an index</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN66552"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>CLUSTER [VERBOSE] <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
> [ USING <TT
CLASS="REPLACEABLE"
><I
>index_name</I
></TT
> ]
CLUSTER [VERBOSE]</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66556"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>CLUSTER</TT
> instructs <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
to cluster the table specified
by <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
>
based on the index specified by
<TT
CLASS="REPLACEABLE"
><I
>index_name</I
></TT
>. The index must
already have been defined on
<TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
>.
</P
><P
> When a table is clustered, it is physically reordered
based on the index information. Clustering is a one-time operation:
when the table is subsequently updated, the changes are
not clustered. That is, no attempt is made to store new or
updated rows according to their index order. (If one wishes, one can
periodically recluster by issuing the command again. Also, setting
the table's <TT
CLASS="LITERAL"
>FILLFACTOR</TT
> storage parameter to less than
100% can aid in preserving cluster ordering during updates, since updated
rows are kept on the same page if enough space is available there.)
</P
><P
> When a table is clustered, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
remembers which index it was clustered by. The form
<TT
CLASS="COMMAND"
>CLUSTER <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
></TT
>
reclusters the table using the same index as before. You can also
use the <TT
CLASS="LITERAL"
>CLUSTER</TT
> or <TT
CLASS="LITERAL"
>SET WITHOUT CLUSTER</TT
>
forms of <A
HREF="sql-altertable.html"
>ALTER TABLE</A
> to set the index to be used for
future cluster operations, or to clear any previous setting.
</P
><P
> <TT
CLASS="COMMAND"
>CLUSTER</TT
> without any parameter reclusters all the
previously-clustered tables in the current database that the calling user
owns, or all such tables if called by a superuser. This
form of <TT
CLASS="COMMAND"
>CLUSTER</TT
> cannot be executed inside a transaction
block.
</P
><P
> When a table is being clustered, an <TT
CLASS="LITERAL"
>ACCESS
EXCLUSIVE</TT
> lock is acquired on it. This prevents any other
database operations (both reads and writes) from operating on the
table until the <TT
CLASS="COMMAND"
>CLUSTER</TT
> is finished.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66579"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
></DT
><DD
><P
> The name (possibly schema-qualified) of a table.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>index_name</I
></TT
></DT
><DD
><P
> The name of an index.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>VERBOSE</TT
></DT
><DD
><P
> Prints a progress report as each table is clustered.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66597"
></A
><H2
>Notes</H2
><P
> In cases where you are accessing single rows randomly
within a table, the actual order of the data in the
table is unimportant. However, if you tend to access some
data more than others, and there is an index that groups
them together, you will benefit from using <TT
CLASS="COMMAND"
>CLUSTER</TT
>.
If you are requesting a range of indexed values from a table, or a
single indexed value that has multiple rows that match,
<TT
CLASS="COMMAND"
>CLUSTER</TT
> will help because once the index identifies the
table page for the first row that matches, all other rows
that match are probably already on the same table page,
and so you save disk accesses and speed up the query.
</P
><P
> <TT
CLASS="COMMAND"
>CLUSTER</TT
> can re-sort the table using either an index scan
on the specified index, or (if the index is a b-tree) a sequential
scan followed by sorting. It will attempt to choose the method that
will be faster, based on planner cost parameters and available statistical
information.
</P
><P
> When an index scan is used, a temporary copy of the table is created that
contains the table data in the index order. Temporary copies of each
index on the table are created as well. Therefore, you need free space on
disk at least equal to the sum of the table size and the index sizes.
</P
><P
> When a sequential scan and sort is used, a temporary sort file is
also created, so that the peak temporary space requirement is as much
as double the table size, plus the index sizes. This method is often
faster than the index scan method, but if the disk space requirement is
intolerable, you can disable this choice by temporarily setting <A
HREF="runtime-config-query.html#GUC-ENABLE-SORT"
>enable_sort</A
> to <TT
CLASS="LITERAL"
>off</TT
>.
</P
><P
> It is advisable to set <A
HREF="runtime-config-resource.html#GUC-MAINTENANCE-WORK-MEM"
>maintenance_work_mem</A
> to
a reasonably large value (but not more than the amount of RAM you can
dedicate to the <TT
CLASS="COMMAND"
>CLUSTER</TT
> operation) before clustering.
</P
><P
> Because the planner records statistics about the ordering of
tables, it is advisable to run <A
HREF="sql-analyze.html"
>ANALYZE</A
>
on the newly clustered table.
Otherwise, the planner might make poor choices of query plans.
</P
><P
> Because <TT
CLASS="COMMAND"
>CLUSTER</TT
> remembers which indexes are clustered,
one can cluster the tables one wants clustered manually the first time,
then set up a periodic maintenance script that executes
<TT
CLASS="COMMAND"
>CLUSTER</TT
> without any parameters, so that the desired tables
are periodically reclustered.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66616"
></A
><H2
>Examples</H2
><P
> Cluster the table <TT
CLASS="LITERAL"
>employees</TT
> on the basis of
its index <TT
CLASS="LITERAL"
>employees_ind</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>CLUSTER employees USING employees_ind;</PRE
><P>
</P
><P
> Cluster the <TT
CLASS="LITERAL"
>employees</TT
> table using the same
index that was used before:
</P><PRE
CLASS="PROGRAMLISTING"
>CLUSTER employees;</PRE
><P>
</P
><P
> Cluster all tables in the database that have previously been clustered:
</P><PRE
CLASS="PROGRAMLISTING"
>CLUSTER;</PRE
><P></P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66627"
></A
><H2
>Compatibility</H2
><P
> There is no <TT
CLASS="COMMAND"
>CLUSTER</TT
> statement in the SQL standard.
</P
><P
> The syntax
</P><PRE
CLASS="SYNOPSIS"
>CLUSTER <TT
CLASS="REPLACEABLE"
><I
>index_name</I
></TT
> ON <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
></PRE
><P>
is also supported for compatibility with pre-8.3 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
versions.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN66636"
></A
><H2
>See Also</H2
><A
HREF="app-clusterdb.html"
><SPAN
CLASS="APPLICATION"
>clusterdb</SPAN
></A
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