Summary:
Continuing from the previous MultiCfIterator Implementations - (https://github.com/facebook/rocksdb/issues/12422, https://github.com/facebook/rocksdb/issues/12480#12465), this PR completes the `AttributeGroupIterator` by implementing `AttributeGroupIteratorImpl::AddToAttributeGroups()`. While implementing the `AttributeGroupIterator`, we had to make some changes in `MultiCfIteratorImpl` and found an opportunity to improve `Coalesce()` in `CoalescingIterator`.
Lifting `UNDER CONSTRUCTION - DO NOT USE` comment by replacing it with `EXPERIMENTAL`
Here are some implementation details:
- `IteratorAttributeGroups` is introduced to avoid having to copy all `WideColumn` objects during iteration.
- `PopulateIterator()` no longer advances non-top iterators that have the same key as the top iterator in the heap.
- `AdvanceIterator()` needs to advance the non-top iterators when they have the same key as the top iterator in the heap.
- Instead of populating one by one, `PopulateIterator()` now collects all items with the same key and calls `populate_func(items)` at once.
- This allowed optimization in `Coalesce()` such that we no longer do K-1 rounds of 2-way merge, but do one K-way merge instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12534
Test Plan:
Uncommented the assertions in `verifyAttributeGroupIterator()`
```
./multi_cf_iterator_test
```
Reviewed By: ltamasi
Differential Revision: D56089019
Pulled By: jaykorean
fbshipit-source-id: 6b0b4247e221f69b40b147d41492008cc9b15054
Summary:
There are a couple of reasons to modify the current implementation of the MultiCfIterator, which implements the generic `Iterator` interface.
- The default behavior of `value()`/`columns()` returning data from different Column Families for different keys can be prone to errors, even though there might be valid use cases where users do not care about the origin of the value/columns.
- The `attribute_groups()` API, which is not yet implemented, will not be useful for a single-CF iterator.
In this PR, we are implementing the following changes:
- `IteratorBase` introduced, which includes all basic iterator functions except `value()` and `columns()`.
- `Iterator`, which now inherits from `IteratorBase`, includes `value()` and `columns()`.
- New public interface `AttributeGroupIterator` inherits from `IteratorBase` and additionally includes `attribute_groups()` (to be implemented).
- Renamed former `MultiCfIterator` to `CoalescingIterator` which inherits from `Iterator`
- Existing MultiCfIteratorTest has been split into two - `CoalescingIteratorTest` and `AttributeGroupIteratorTest`.
- Moved AttributeGroup related code from `wide_columns.h` to a new file, `attribute_groups.h`.
Some Implementation Details
- `MultiCfIteratorImpl` takes two functions - `populate_func` and `reset_func` and use them to populate `value_` and `columns_` in CoalescingIterator and `attribute_groups_` in AttributeGroupIterator. In CoalescingIterator, populate_func is `Coalesce()`, in AttributeGroupIterator populate_func is `AddToAttributeGroups()`. `reset_func` clears populated value_, columns_ and attribute_groups_ accordingly.
- `Coalesce()` merge sorts columns from multiple CFs when a key exists in more than on CFs. column that appears in later CF overwrites the prior ones.
For example, if CF1 has `"key_1" ==> {"col_1": "foo", "col_2", "baz"}` and CF2 has `"key_1" ==> {"col_2": "quux", "col_3", "bla"}`, and when the iterator is at `key_1`, `columns()` will return `{"col_1": "foo", "col_2", "quux", "col_3", "bla"}`
In this example, `value()` will be empty, because none of them have values for `kDefaultColumnName`
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12480
Test Plan:
## Unit Test
```
./multi_cf_iterator_test
```
## Performance Test
To make sure this change does not impact existing `Iterator` performance
**Build**
```
$> make -j64 release
```
**Setup**
```
$> TEST_TMPDIR=/dev/shm/db_bench ./db_bench -benchmarks="filluniquerandom" -key_size=32 -value_size=512 -num=1000000 -compression_type=none
```
**Run**
```
TEST_TMPDIR=/dev/shm/db_bench ./db_bench -use_existing_db=1 -benchmarks="newiterator,seekrandom" -cache_size=10485760000
```
**Before the change**
```
DB path: [/dev/shm/db_bench/dbbench]
newiterator : 0.519 micros/op 1927904 ops/sec 0.519 seconds 1000000 operations;
DB path: [/dev/shm/db_bench/dbbench]
seekrandom : 5.302 micros/op 188589 ops/sec 5.303 seconds 1000000 operations; (0 of 1000000 found)
```
**After the change**
```
DB path: [/dev/shm/db_bench/dbbench]
newiterator : 0.497 micros/op 2011012 ops/sec 0.497 seconds 1000000 operations;
DB path: [/dev/shm/db_bench/dbbench]
seekrandom : 5.252 micros/op 190405 ops/sec 5.252 seconds 1000000 operations; (0 of 1000000 found)
```
Reviewed By: ltamasi
Differential Revision: D55353909
Pulled By: jaykorean
fbshipit-source-id: 8d7786ffee09e022261ce34aa60e8633685e1946
