Nan Zhu created SPARK-22599:
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Summary: Avoid extra reading for cached table
Key: SPARK-22599
URL: https://issues.apache.org/jira/browse/SPARK-22599
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.2.0
Reporter: Nan Zhu
In the current implementation of Spark, InMemoryTableExec read all data in a
cached table, filter CachedBatch according to stats and pass data to the
downstream operators. This implementation makes it inefficient to reside the
whole table in memory to serve various queries against different partitions of
the table, which occupies a certain portion of our users' scenarios.
The following is an example of such a use case:
store_sales is a 1TB-sized table in cloud storage, which is partitioned by
'location'. The first query, Q1, wants to output several metrics A, B, C for
all stores in all locations. After that, a small team of 3 data scientists
wants to do some causal analysis for the sales in different locations. To avoid
unnecessary I/O and parquet/orc parsing overhead, they want to cache the whole
table in memory in Q1.
With the current implementation, even any one of the data scientists is only
interested in one out of three locations, the queries they submit to Spark
cluster is still reading 1TB data completely.
The reason behind the extra reading operation is that we implement CachedBatch
as
{code:scala}
case class CachedBatch(numRows: Int, buffers: Array[Array[Byte]], stats:
InternalRow)
{code}
where "stats" is a part of every CachedBatch, so we can only filter batches for
output of InMemoryTableExec operator by reading all data in in-memory table as
input. The extra reading would be even more unacceptable when some of the
table's data is evicted to disks.
We propose to introduce a new type of block, metadata block, for the partitions
of RDD representing data in the cached table. Every metadata block contains
stats info for all columns in a partition and is saved to BlockManager when
executing compute() method for the partition. To minimize the number of bytes
to read,
More details can be found in design
doc:https://docs.google.com/document/d/1DSiP3ej7Wd2cWUPVrgqAtvxbSlu5_1ZZB6m_2t8_95Q/edit?usp=sharing
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