Re: Air-traffic benchmark - Mailing list pgsql-performance

----- "Matthew Wakeling" <matthew@flymine.org> escreveu:

> On Thu, 7 Jan 2010, Gurgel, Flavio wrote:

> Postgres does not change a query plan according to the shared_buffers
>
> setting. It does not anticipate one step contributing to another step
> in
> this way. It does however make use of the effective_cache_size setting
> to
> estimate this effect, and that does affect the planner.

That was what I was trying to say :)

> In a situation like this, the opposite will be true. If you were
> accessing
> a very small part of a table, say to order by a field with a small
> limit,
> then an index can be very useful by providing the results in the
> correct
> order. However, in this case, almost the entire table has to be read.
>
> Changing the order in which it is read will mean that the disc access
> is
> no longer sequential, which will slow things down, not speed them up.
>
> The Postgres planner isn't stupid (mostly), there is probably a good
> reason why it isn't using an index scan.

Sorry but I disagree. This is the typical case where the test has to be made.
The results are partial, let's say 50% of the table. Considerind that the disk is fast enough, the cost estimation of sequential and random reads are in a proportion of 1 to 4, considering default settings in PostgreSQL.

If, and only if the data is time distributed in the table (which can be this case during bulk load) there will be some gain in seqscan.
If, let's say 50% of the 50% (25% of the data) is time distributed (which can be the case in most data warehouses), the cost of random reads * number of reads can be cheaper then seqscan.

The volume of data doesn't turn the index generations so deep, let's say 2^7 or 2^8. This can lead to very fast data retrieval.

> > Ugh. I don't think so. That's why indexes were invented. PostgreSQL
> is
> > smart enough to "jump" over columns using byte offsets.
> > A better option for this table is to partition it in year (or
> year/month) chunks.
>
> Postgres (mostly) stores the columns for a row together with a row, so
>
> what you say is completely wrong. Postgres does not "jump" over
> columns
> using byte offsets in this way. The index references a row in a page
> on
> disc, and that page is fetched separately in order to retrieve the
> row.
> The expensive part is physically moving the disc head to the right
> part of
> the disc in order to fetch the correct page from the disc - jumping
> over
> columns will not help with that at all.

If the index point to the right pages, I keep the circumstance of 1 to 4 cost.
Agreed about seqscans. When I talked about byte offsets I was talking of data in the same disk page, and this does not help I/O reduction at all.

> Reducing the width of the table will greatly improve the performance
> of a
> sequential scan, as it will reduce the size of the table on disc, and
>
> therefore the time taken to read the entire table sequentially.

I just don't understand if you're seeing this situation as OLTP or DW, sorry.
DW tables are usually wider then OLTP.

> Moreover, your suggestion of partitioning the table may not help much
> with
> this query. It will turn a single sequential scan into a UNION of many
>
> tables, which may be harder for the planner to plan. Also, for queries

Partitioned plans are a collection of an independent plan for each table in the inheritance.
If the data to be retrieved is confined in selected partitions, you won't seqscan the partitions you don't need.
The cost of the "union" of the aggregations in memory is a lot cheaper then the avoided seqscans.

I have at least 3 cases of partitioning in queries exactly like this that droped from 3min to 5s execution times.
All of that DW tables, with aggregation and huge seqscans.

I keep my word that the right use of indexes here has to be tested.

Flavio Henrique A. Gurgel
tel. 55-11-2125.4786
cel. 55-11-8389.7635

www.4linux.com.br
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