Re: Batch update of indexes

On 21.01.2016 19:09, Anastasia Lubennikova wrote:
> What I meant is more like a BRIN-like combination of an index scan and
> heap scan.
> Maybe it could be called "deferred inserts" or "temporary read-only
> index"
> Maybe it's similar with mysql insert buffer
> http://dev.mysql.com/doc/refman/5.7/en/innodb-insert-buffering.html
> I think it'll be more clear with example. Please don't care about syntax.
>
> CREATE TABLE tbl (c1 int);
> CREATE INDEX idx on tbl(c1);
>
> SET enable_deferred_insert(idx) = on;
> At this moment, we save the last_indexed_item (its TID) somewhere in
> index metapage.
>
> Since that moment, the data inserted into the table doesn't touch the
> index.
> We perform some heavy insert and then go back to the normal index
> behavior.
>
> SET enable_deferred_insert(idx) = off;
> This command takes all the data between the last_indexed_item and the
> end of the table, and inserts it into the index at a time.
>
> Of course there are new problems to deal with, but it's really useful
> for the use case to balance irregular heavy write load, isn't it?
>
> BTW, could you explain, what is the reason to copy data into the
> pending list and then copy it again while flushing pending list into
> the index? Why not read this data directly from the table? I feel that
> I've missed something important here.
>
No, I do not think that inserted data should be placed in pending list
and then copied to main table.
It should be stored directly in the main table and "pending list" is
just some fast, transient index.

From my point of view there are two possibilities:
1. Preserve strict table-index consistency: query results should not
depend on presence of the index
2. Support out-of-date or deferred indexes, which can be updated in
background.

Second approach is certainty more efficient and IMHO it acceptable for
most of the users.
But we are violating one of the fundamental properties of RDBMes...
So I am not sure which approach to chose.

First case is also harder to implement, because we have to somehow merge
two indexes during index scan
and provide proper recovery of main index in case of failure (assuming
that pending list is maintained in memory and is lost after the fault).

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company