1CLUSTER() SQL Commands CLUSTER()
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6 CLUSTER - cluster a table according to an index
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10 CLUSTER indexname ON tablename
11 CLUSTER tablename
12 CLUSTER
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16 CLUSTER instructs PostgreSQL to cluster the table specified by table‐
17 name based on the index specified by indexname. The index must already
18 have been defined on tablename.
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20 When a table is clustered, it is physically reordered based on the
21 index information. Clustering is a one-time operation: when the table
22 is subsequently updated, the changes are not clustered. That is, no
23 attempt is made to store new or updated rows according to their index
24 order. If one wishes, one can periodically recluster by issuing the
25 command again.
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27 When a table is clustered, PostgreSQL remembers on which index it was
28 clustered. The form CLUSTER tablename reclusters the table on the same
29 index that it was clustered before.
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31 CLUSTER without any parameter reclusters all the tables in the current
32 database that the calling user owns, or all tables if called by a supe‐
33 ruser. (Never-clustered tables are not included.) This form of CLUSTER
34 cannot be executed inside a transaction block.
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36 When a table is being clustered, an ACCESS EXCLUSIVE lock is acquired
37 on it. This prevents any other database operations (both reads and
38 writes) from operating on the table until the CLUSTER is finished.
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41 indexname
42 The name of an index.
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44 tablename
45 The name (possibly schema-qualified) of a table.
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48 CLUSTER loses all visibility information of tuples, which makes the ta‐
49 ble look empty to any snapshot that was taken before the CLUSTER com‐
50 mand finished. That makes CLUSTER unsuitable for applications where
51 transactions that access the table being clustered are run concurrently
52 with CLUSTER. This is most visible with serializable transactions,
53 because they take only one snapshot at the beginning of the transac‐
54 tion, but read-committed transactions are also affected.
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56 In cases where you are accessing single rows randomly within a table,
57 the actual order of the data in the table is unimportant. However, if
58 you tend to access some data more than others, and there is an index
59 that groups them together, you will benefit from using CLUSTER. If you
60 are requesting a range of indexed values from a table, or a single
61 indexed value that has multiple rows that match, CLUSTER will help
62 because once the index identifies the table page for the first row that
63 matches, all other rows that match are probably already on the same ta‐
64 ble page, and so you save disk accesses and speed up the query.
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66 During the cluster operation, a temporary copy of the table is created
67 that contains the table data in the index order. Temporary copies of
68 each index on the table are created as well. Therefore, you need free
69 space on disk at least equal to the sum of the table size and the index
70 sizes.
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72 Because CLUSTER remembers the clustering information, one can cluster
73 the tables one wants clustered manually the first time, and setup a
74 timed event similar to VACUUM so that the tables are periodically
75 reclustered.
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77 Because the planner records statistics about the ordering of tables, it
78 is advisable to run ANALYZE [analyze(7)] on the newly clustered table.
79 Otherwise, the planner may make poor choices of query plans.
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81 There is another way to cluster data. The CLUSTER command reorders the
82 original table by scanning it using the index you specify. This can be
83 slow on large tables because the rows are fetched from the table in
84 index order, and if the table is disordered, the entries are on random
85 pages, so there is one disk page retrieved for every row moved. (Post‐
86 greSQL has a cache, but the majority of a big table will not fit in the
87 cache.) The other way to cluster a table is to use
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89 CREATE TABLE newtable AS
90 SELECT * FROM table ORDER BY columnlist;
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92 which uses the PostgreSQL sorting code to produce the desired order;
93 this is usually much faster than an index scan for disordered data.
94 Then you drop the old table, use ALTER TABLE ... RENAME to rename
95 newtable to the old name, and recreate the table's indexes. The big
96 disadvantage of this approach is that it does not preserve OIDs, con‐
97 straints, foreign key relationships, granted privileges, and other
98 ancillary properties of the table — all such items must be manually
99 recreated. Another disadvantage is that this way requires a sort tempo‐
100 rary file about the same size as the table itself, so peak disk usage
101 is about three times the table size instead of twice the table size.
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104 Cluster the table employees on the basis of its index emp_ind:
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106 CLUSTER emp_ind ON emp;
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109 Cluster the employees table using the same index that was used before:
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111 CLUSTER emp;
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114 Cluster all tables in the database that have previously been clustered:
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116 CLUSTER;
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120 There is no CLUSTER statement in the SQL standard.
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123 clusterdb [clusterdb(1)]
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127SQL - Language Statements 2008-06-08 CLUSTER()