1CLUSTER(7) PostgreSQL 15.4 Documentation CLUSTER(7)
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6 CLUSTER - cluster a table according to an index
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9 CLUSTER [VERBOSE] table_name [ USING index_name ]
10 CLUSTER ( option [, ...] ) table_name [ USING index_name ]
11 CLUSTER [VERBOSE]
12 where option can be one of:
14 VERBOSE [ boolean ]
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18 CLUSTER instructs PostgreSQL to cluster the table specified by
19 table_name based on the index specified by index_name. The index must
20 already have been defined on table_name.
21 When a table is clustered, it is physically reordered based on the
23 index information. Clustering is a one-time operation: when the table
24 is subsequently updated, the changes are not clustered. That is, no
25 attempt is made to store new or updated rows according to their index
26 order. (If one wishes, one can periodically recluster by issuing the
27 command again. Also, setting the table's fillfactor storage parameter
28 to less than 100% can aid in preserving cluster ordering during
29 updates, since updated rows are kept on the same page if enough space
30 is available there.)
31 When a table is clustered, PostgreSQL remembers which index it was
33 clustered by. The form CLUSTER table_name reclusters the table using
34 the same index as before. You can also use the CLUSTER or SET WITHOUT
35 CLUSTER forms of ALTER TABLE to set the index to be used for future
36 cluster operations, or to clear any previous setting.
37 CLUSTER without any parameter reclusters all the previously-clustered
39 tables in the current database that the calling user owns, or all such
40 tables if called by a superuser. This form of CLUSTER cannot be
41 executed inside a transaction block.
42 When a table is being clustered, an ACCESS EXCLUSIVE lock is acquired
44 on it. This prevents any other database operations (both reads and
45 writes) from operating on the table until the CLUSTER is finished.
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48 table_name
49 The name (possibly schema-qualified) of a table.
50 index_name
52 The name of an index.
53 VERBOSE
55 Prints a progress report as each table is clustered.
56 boolean
58 Specifies whether the selected option should be turned on or off.
59 You can write TRUE, ON, or 1 to enable the option, and FALSE, OFF,
60 or 0 to disable it. The boolean value can also be omitted, in which
61 case TRUE is assumed.
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64 In cases where you are accessing single rows randomly within a table,
65 the actual order of the data in the table is unimportant. However, if
66 you tend to access some data more than others, and there is an index
67 that groups them together, you will benefit from using CLUSTER. If you
68 are requesting a range of indexed values from a table, or a single
69 indexed value that has multiple rows that match, CLUSTER will help
70 because once the index identifies the table page for the first row that
71 matches, all other rows that match are probably already on the same
72 table page, and so you save disk accesses and speed up the query.
73 CLUSTER can re-sort the table using either an index scan on the
75 specified index, or (if the index is a b-tree) a sequential scan
76 followed by sorting. It will attempt to choose the method that will be
77 faster, based on planner cost parameters and available statistical
78 information.
79 When an index scan is used, a temporary copy of the table is created
81 that contains the table data in the index order. Temporary copies of
82 each index on the table are created as well. Therefore, you need free
83 space on disk at least equal to the sum of the table size and the index
84 sizes.
85 When a sequential scan and sort is used, a temporary sort file is also
87 created, so that the peak temporary space requirement is as much as
88 double the table size, plus the index sizes. This method is often
89 faster than the index scan method, but if the disk space requirement is
90 intolerable, you can disable this choice by temporarily setting
91 enable_sort to off.
92 It is advisable to set maintenance_work_mem to a reasonably large value
94 (but not more than the amount of RAM you can dedicate to the CLUSTER
95 operation) before clustering.
96 Because the planner records statistics about the ordering of tables, it
98 is advisable to run ANALYZE on the newly clustered table. Otherwise,
99 the planner might make poor choices of query plans.
100 Because CLUSTER remembers which indexes are clustered, one can cluster
102 the tables one wants clustered manually the first time, then set up a
103 periodic maintenance script that executes CLUSTER without any
104 parameters, so that the desired tables are periodically reclustered.
105 Each backend running CLUSTER will report its progress in the
107 pg_stat_progress_cluster view. See Section 28.4.4 for details.
108 Clustering a partitioned table clusters each of its partitions using
110 the partition of the specified partitioned index. When clustering a
111 partitioned table, the index may not be omitted.
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114 Cluster the table employees on the basis of its index employees_ind:
115 CLUSTER employees USING employees_ind;
117 Cluster the employees table using the same index that was used before:
119 CLUSTER employees;
121 Cluster all tables in the database that have previously been clustered:
123 CLUSTER;
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127 There is no CLUSTER statement in the SQL standard.
128 The syntax
130 CLUSTER index_name ON table_name
132 is also supported for compatibility with pre-8.3 PostgreSQL versions.
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136 clusterdb(1), Section 28.4.4
137PostgreSQL 15.4 2023 CLUSTER(7)