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From Stack <st...@duboce.net>
Subject Re: Row+Col Range Read/Scan
Date Wed, 10 Aug 2011 18:33:36 GMT
On Wed, Aug 10, 2011 at 2:39 AM, Wayne <wav100@gmail.com> wrote:
> As we load more and more data into HBase we are seeing the "millions of
> columns" to be a challenge for us. We have some very wide rows and we are
> taking 12-15 seconds to read those rows.

How many columns when its taking this long Wayne?


> Since HBase does not sort columns

They are sorted.

> and thereby can not support a scan of columns

How do you mean?  You only want a subset of the columns?  Can you add
a filter or add some subset of the columns to the Scan specification?

You can also read a piece of the row only if that is all you are
interested in (though you are on other side of thrift, right, and this
facility may not be exposed -- I have not checked)

> Is/has there been any talk about building in some support for sorted columns
> and the ability to read/scan across columns? Millions of columns are
> challenging if you can only read a single column/list of columns or the
> entire thing.

When you say read/scan across columns, can you say more what you'd
like?  You'd like to read N columns at a time?

> How does bigtable support this? It seems that hbase is limited
> as a column based data store unless it can support this. Our columns are
> truly dynamic so we do not even necessarily know what they are to request
> them by name in a list. We want to be able to read/scan them just like for
> rows.
>

In java you'd do
http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html#setBatch(int)


> We would love the ability to support the following read method (through
> Thrift). We can of course do this on our own from the entire row but it
> requires reading the 2 million col row into memory first.
>

How big are the cells?  How big is the 2M row?  You don't know the
name but do they fit a pattern that you could filter on?  (Though
again, filters are not exposed in thrift though that looks like its
getting fixed)

> getRowWithColumnRange(tableName, row, startColumn, stopColumn)
>
> The above would be even better if it could be set up like a scanner where we
> could stop at any point. Basically instead of scanning rows we would scan
> columns for a given row. This would be the best way to support an offset,
> limit pattern.
>
> colScanID = colScannerOpenWithStop(tableName, row, startColumn, stopColumn)
> colScannerGetList (colSanID,1000)
>
> Of course once these changes occurred people would be pushing the size of
> rows even more. We have seen somewhere around 20+ million columns cause OOM
> errors. One row per region should be the theoretical limit to the row size,
> but there is more work needed I am sure to ensure that this is true.
>

The above look useful.  Stick them into an issue Wayne.

St.Ack
P.S. I'm still working (slowly) on the recover tool you asked for in
your last mail.

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