Use Scanner API for all reads
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Key: CASSANDRA-1106
URL: https://issues.apache.org/jira/browse/CASSANDRA-1106
Project: Cassandra
Issue Type: Sub-task
Reporter: Stu Hood
Priority: Minor
Fix For: 0.8
The goal of this issue is to eliminate the IColumnIterator interface, and to
use the Slice/Scanner API for all reads. Additionally, this issue begins to
optimize the interaction between FilteredScanner and QueryFilter to gain back
speed lost in CASSANDRA-1095.
This issue adds Memtable.Scanner and converts Memtables to maps from
DecoratedKey -> List<Slice> (where the list represents a row: one entry for
Standard CFs, and more than one entry for Super CFs). Since Slices are
immutable, rows in the Memtable are merged using SliceMergingIterator, and
atomically swapped out. This is much less granular atomicity than we support
currently, so this approach to mapping the Memtable to Slices is wide open to
debate.
The row cache in this patch mimics the Memtable and becomes a map from
DecoratedKey -> List<Slice>. In order to reuse the QueryFilter API, a
db.ListScanner is added to wrap an individual row in the cache for filtering.
One limitation imposed by this design is that the row cache can't be used as a
write-through cache, since its entries are immutable.
The common order of operations is:
# Get a SeekableScanner implementation for the Memtable/cache entry/SSTable
# Build a QueryFilter describing the query
# Call QueryFilter.filter(scanner) to wrap the SeekableScanner in a
FilteredScanner
* Optionally, merge multiple Scanners using MergingScanner
# Call QueryFilter.collect(scanner) to wrap garbage collection around the
merged input
# Limit the output columns using QueryFilter.limit(scanner)
Optimization between FilteredScanner and QueryFilter is accomplished via the
MatchResult object, which is pretty ugly, and still a work in progress.
Internally to a QueryFilter, IFilters for each level return MatchResults
indicating where their next interesting matches are, and QueryFilter composes
the levels into a MathResult that a FilteredScanner uses to see on its
underlying Scanner.
These patches remove a lot of deeply nested and complicated logic for dealing
with super columns and garbage collection, including IFilter.filterSuperColumn
(replaced naturally by Slice filtering), IFilter.collectReducedColumns (ditto)
and ColumnFamilyStore.removeDeleted (replaced by ASlice.GCFunction).
Additionally, they replace scads of AbstractIterator implementations that were
implementing IColumnIterator on a case by case basis.
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