diff options
| -rw-r--r-- | doc/src/sgml/ref/create_index.sgml | 17 |
1 files changed, 10 insertions, 7 deletions
diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml index ce36a1ba480..745089993be 100644 --- a/doc/src/sgml/ref/create_index.sgml +++ b/doc/src/sgml/ref/create_index.sgml @@ -426,7 +426,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class= When this option is used, <productname>PostgreSQL</> must perform two scans of the table, and in addition it must wait for all existing transactions that could potentially - use the index to terminate. Thus + modify or use the index to terminate. Thus this method requires more total work than a standard index build and takes significantly longer to complete. However, since it allows normal 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= <para> In a concurrent index build, the index is actually entered into the system catalogs in one transaction, then two table scans occur in - two more transactions. Any transaction active when the second table - scan starts can block concurrent index creation until it completes, - even transactions that only reference the table after the second table - scan starts. Concurrent index creation serially waits for each old - transaction to complete using the method outlined in section <xref - linkend="view-pg-locks">. + two more transactions. Before each table scan, the index build must + wait for existing transactions that have modified the table to terminate. + After the second scan, the index build must wait for any transactions + that have a snapshot (see <xref linkend="mvcc">) predating the second + scan to terminate. Then finally the index can be marked ready for use, + and the <command>CREATE INDEX</> command terminates. + Even then, however, the index may not be immediately usable for queries: + in the worst case, it cannot be used as long as transactions exist that + predate the start of the index build. </para> <para> |