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Commit6ce8236

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Improve documentation about CREATE INDEX CONCURRENTLY.
Clarify the description of which transactions will block a CREATE INDEXCONCURRENTLY command from proceeding, and mention that the index mightstill not be usable after CREATE INDEX completes. (This happens if theindex build detected broken HOT chains, so that pg_index.indcheckxmin getsset, and there are open old transactions preventing the xmin horizon fromadvancing past the index's initial creation. I didn't want to explain whatbroken HOT chains are, though, so I omitted an explanation of exactly whenold transactions prevent the index from being used.)Per discussion with Chris Travers. Back-patch to all supported branches,since the same text appears in all of them.
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‎doc/src/sgml/ref/create_index.sgml‎

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@@ -426,7 +426,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
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When this option is used,
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<productname>PostgreSQL</> must perform two scans of the table, and in
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addition it must wait for all existing transactions that could potentially
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use the index to terminate. Thus
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modify oruse the index to terminate. Thus
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this method requires more total work than a standard index build and takes
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significantly longer to complete. However, since it allows normal
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operations to continue while the index is built, this method is useful for
@@ -437,12 +437,15 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
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<para>
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In a concurrent index build, the index is actually entered into
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the system catalogs in one transaction, then two table scans occur in
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two more transactions. Any transaction active when the second table
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scan starts can block concurrent index creation until it completes,
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even transactions that only reference the table after the second table
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scan starts. Concurrent index creation serially waits for each old
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transaction to complete using the method outlined in section <xref
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linkend="view-pg-locks">.
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two more transactions. Before each table scan, the index build must
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wait for existing transactions that have modified the table to terminate.
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After the second scan, the index build must wait for any transactions
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that have a snapshot (see <xref linkend="mvcc">) predating the second
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scan to terminate. Then finally the index can be marked ready for use,
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and the <command>CREATE INDEX</> command terminates.
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Even then, however, the index may not be immediately usable for queries:
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in the worst case, it cannot be used as long as transactions exist that
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predate the start of the index build.
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</para>
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<para>

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