Re: INVALID index while concurrent indexing in progress? - Mailing list pgsql-docs

Thank you both! I think Laurenz’s changes make perfect sense!

Sent from my iPhone

> On May 19, 2022, at 8:37 AM, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> On Thu, 2022-05-19 at 19:02 +0530, Rajakavitha Kodhandapani wrote:
>>> I think the INVALID index can use further explanation, in particular, from
>>> my experience it seems like when building an index concurrently, the index
>>> gets inserted and labeled invalid while the index is being built, and then
>>> the label gets removed if it finishes successfully or gets left on the index
>>> if there is a failure while building the index.  It is my current
>>> understanding, after experimenting, that INVALID means 'incomplete', whether
>>> that's because it's in progress or because it was unable to be completed,
>>> but prior to my experiment my understanding was that INVALID indicated
>>> failure.
>>>
>>> This was especially confusing when we were adding an index to a very large
>>> table because we assumed the INVALID index indicated failure when we
>>> couldn't find any other sign of progress or failure.
>>
>> This is my first attempt at contributing to the documentation of PostgreSQL.
>> Here's the patch. Please let me know if any other changes need to be made.
>
> Thank you!  Please send patches as plain text and use bottom posting.
>
>> \cf3 @@ -665,11 +665,14 @@\cf5  Indexes:\
>>    <para>\
>>     Another caveat when building a unique index concurrently is that the\
>>     uniqueness constraint is already being enforced against other transactions\
>> \cf6 -    when the second table scan begins.  This means that constraint violations\cf5 \
>> \cf4 +    when the second table scan begins. This means that constraint violations\cf5 \
>>     could be reported in other queries prior to the index becoming available\
>> \cf6 -    for use, or even in cases where the index build eventually fails.  Also,\cf5 \
>> \cf6 -    if a failure does occur in the second scan, the <quote>invalid</quote> index\cf5 \
>> \cf6 -    continues to enforce its uniqueness constraint afterwards.\cf5 \
>> \cf4 +    for use, or even in cases where the index build eventually fails. The index\cf5 \
>> \cf4 +    is inserted and labeled <quote>invalid</quote> while the index is being built,\cf5 \
>> \cf4 +    and then the label is removed if the index builds successfully. If the index does\cf5 \
>> \cf4 +    not build successfully, then the label <quote>invalid</quote> remains. Also, if a\cf5 \
>> }
>
> I don't think that this information should be added to a paragraph that
> focuses on uniqueness checks in concurrent index builds.
>
> Actually, most of the information is already there.  To quote from the page:
>
>  If a problem arises while scanning the table, such as a deadlock or a uniqueness violation
>  in a unique index, the CREATE INDEX command will fail but leave behind an “invalid” index.
>
> How about the following patch to emphasize the role of "invalid":
>
> diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
> index d3102a87d9..fee2c61e5e 100644
> --- a/doc/src/sgml/ref/create_index.sgml
> +++ b/doc/src/sgml/ref/create_index.sgml
> @@ -622,7 +622,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
>    </para>
>
>    <para>
> -    In a concurrent index build, the index is actually entered into
> +    In a concurrent index build, the index is actually entered as <quote>invalid</quote> index into
>     the system catalogs in one transaction, then two table scans occur in
>     two more transactions.  Before each table scan, the index build must
>     wait for existing transactions that have modified the table to terminate.
> @@ -631,7 +631,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
>     scan to terminate, including transactions used by any phase of concurrent
>     index builds on other tables, if the indexes involved are partial or have
>     columns that are not simple column references.
> -    Then finally the index can be marked ready for use,
> +    Then finally the index can be marked <quote>valid</quote> and ready for use,
>     and the <command>CREATE INDEX</command> 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
>
> Yours,
> Laurenz Albe