[PHOENIX-111] Improve intra-region parallelization

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Duplicate
• Assignee: Lars Hofhansl
• Reporter: James R. Taylor
• Created: 05/Mar/14 20:36
• Updated: 05/Aug/14 22:32
• Resolved: 05/Aug/14 22:32

Description

The manner in which Phoenix parallelizes queries is explained in some detail in the Parallelization section here: http://phoenix-hbase.blogspot.com/2013/02/phoenix-knobs-dials.html

It's actually not that important to understand all the details. In the case where we try to parallelize within a region, we rely on the HBase Bytes.split() method (in DefaultParallelIteratorRegionSplitter) to split, based on the start and end key of the region. We basically use that method to come up with the start row and stop row of scans that will all run in parallel across that region.

The problem is, we haven't really tested this method, and I have my doubts about it, especially when two keys are of different length. The first thing that should be done is to write a few simple, independent tests using Bytes.split() directly to confirm whether or not there's a problem:

1. Write some simple tests to see if Bytes.split() works as expected. Does it work for two keys that are of different lengths? If not, we can likely take two keys and make them the same length through padding b/c we know the structure of the row key. The better we choose the split points to get even distribution, the better our parallelization will be.
2. One case that I know will be problematic is when a table is salted. In that case, we pre-split the table into N regions, where N is the SALT_BUCKETS=<N> value. The problem in this case is that the Bytes.split() points are going to be terrible, because it's not taking into account the possible values of the row key. For example, imagine you have a table like this:

CREATE TABLE foo(k VARCHAR PRIMARY KEY) SALT_BUCKETS=4

In this case, we'll pre-split the table and have the following region boundaries: 0-1, 1-2, 2-3, 3-4

What will be the Bytes.split() for these region boundaries? It would chunk it up into even byte boundaries which is not ideal, because the VARCHAR value would most likely be ascii characters in a range of 'A' to 'z'. We'd be much better off if we took into account the data types of the row key when we calculate these split points.

So the second thing to do is make some simple improvements to the start/stop key we pass Bytes.split() that take into account the data type of each column that makes up the primary key.

For Phoenix 5.0, we'll collect stats and drive this off of those, but for now, there's likely a few simple things we could do to make a big improvement.