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Expand warnings on locks acquired by CREATE INDEX CONCURRENTLY

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The previous wording wasn't explicit enough, which could misled readers
into thinking that the locks acquired are more restricted in nature than
they really are.  The resulting optimism can be damaging to morale when
confronted with reality, as has been observed in the field.

Greg Smith
This commit is contained in:
Alvaro Herrera 2011-06-13 17:12:26 -04:00
parent 2202891669
commit a03feb9354
1 changed files with 10 additions and 1 deletions
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doc/src/sgml/ref/create_index.sgml
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@ -381,7 +381,16 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ <replaceable class="parameter">name</
<para>
In a concurrent index build, the index is actually entered into the
system catalogs in one transaction, then the two table scans occur in a
second and third transaction.
second and third transaction. All active transactions at the time the
second table scan starts, not just ones that already involve the table,
have the potential to block the concurrent index creation until they
finish. When checking for transactions that could still use the original
index, concurrent index creation advances through potentially interfering
older transactions one at a time, obtaining shared locks on their virtual
transaction identifiers to wait for them to complete.
</para>
<para>
If a problem arises while scanning the table, such as a
uniqueness violation in a unique index, the <command>CREATE INDEX</>
command will fail but leave behind an <quote>invalid</> index. This index