If you just want the first 10 by a certain prefix ordered by a
column, then you will definitely be better off scanning them by row
and ordering them clientside.
Surely your idea of maintaining a separate SortedMap in each region
would work, but I don't think you should discount the cost associated
with having to talk to a bunch of different regions every time you
want to know what the next row is. You'll do a lot of extra work to
get that merged view of the index, and potentially the approach won't
scale up to queries that might cover more than a "few" regions - can
you imagine having to check 100 or 1000 regions for the next result
every time you needed to iterate?
On Apr 22, 2008, at 12:58 PM, Clint Morgan wrote:
Yeah, that would be an easy approach. We would need HBASE-82.
The main problem I see here is that we cannot take (as much) advantage
of our row key prefix in weeding out rows.
Say I want the first 10 rows that start with XXX ordered by A:amount,
then I would have scan through column values from rows everywhere in
the table until I hit 10 with my prefix. Could be costly if table is
large compared to the number of rows that start with XXX.
Whereas if we have one SortedMap per Region, then I can quickly narrow
down to (hopefully) a few regions based on key prefix.
Though other usage / table loading patterns would surely benefit from
this approach...
On Tue, Apr 22, 2008 at 12:31 PM, Bryan Duxbury <[EMAIL PROTECTED]>
wrote:
This doesn't have to be all that complicated.
Why not keep another HBase table as the index? The keys would be
the column
values. There'd be a single matchingRows column family, and the
qualifiers
and values would be the rows that match that column. Then, when
you want to
scan in column order instead of row order, you scan the index
table, find
the list of rows that match each column, and then do a random read
to grab
those individually. It'll for sure be slower than scanning a table
ordered
by rows, but it'll get you what you want. It'll also handle the
case where
the column values aren't unique.
If you need custom sorting for those values, then HBASE-82 would
solve that
problem.
-Bryan
On Apr 22, 2008, at 11:58 AM, stack wrote:
Some questions interlaced below:
Clint Morgan wrote:
All,
We want to put secondary indexes into hbase. The motivation is
that we
are storing data in hbase that we want to serve to users. We would
like to be able to serve rows sorted by column values. Our queries
will be over rows with a given key prefix, so we should not be
hitting
to many regions.
I was thinking it would work roughly like this:
- At table creation time, individual columns can be declared as
indexed. By default we could sort the column values
lexicographically,
or we can provide a WritableComparatorFactory<T> which has the
ability
to make values of type T from a byte [], as well as providing a
Comparator<T>. (Better than providing a Comparator<byte[]> as it
only
costs once per row insert for deserialization, rather that twice on
each comparison).
I don't follow what the Factory adds.
We're talking about getting HBASE-82 into 0.2. Does that
interfere with
this proposal? (I'm thinking that along w/ rows becoming byte
arrays rather
Text with a client-supplied Comparator, column qualifiers would
shift to be
byte arrays also -- though yeah, implies that if your sort is not
byte-lexicographical, yes, the compares can be costly involving two
deserializations per compare).
- We catch all writes/deletes and maintain a SortedMap<T,
HStoreKey>
which keeps the column values in order, and maps them back to row
keys. First cut may just keep all this in memory, but it should be
backed with MapFile(s).
Would be sweet if you could leverage the HBase memcache code and
flusher
to do the above.
This Map would be global for the table? Or per Region?
A lucene index wouldn't work for you because you want ordering?
- Add to the hregion the ability to scan through keys in column
order.
Just iterate through the SortedMap, run a filter on the key, and
if it
passes do a get on the row.
You'd be random reading rows. You're OK w/ current performance?
(For
sure it will only improve but....).
- Add a ColumnOrderedClientScanner which will open column order
scanners to all applicable hregions, and continuously pick row with
the lowest column value from each of the client scanners.
This scanner would have a significant client-side component to do
the
arbitrage between all regions to figure the lowest column value?
If you had
a new type of 'region' -- one denoted by lowest and upper column
then the
client-side logic would fade away and your scanner would look like
current
scanners.
- Region splits should be easy enough, just a scan through the
SortedMap to partition.
Splits would not be row-based and run as they currently do, but
rather
sorted-column based?
Of course, the index could also be used for more efficient
querying on
the indexed column's values.
Do other users have a need for this functionality?
What do developers think about this? I know hbase is more
intended for
back-end batch style processing, but we have this need.
How are you thinking of adding in this new functionality?
Subclassing
HRegionServer?
St.Ack
Cheers,
-clint
