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Add comments to db/db_iter.h (#5340)

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Summary:
Add file comment in db/db_iter.h and minor changes in other parts.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5340

Differential Revision: D15484605

Pulled By: siying

fbshipit-source-id: 173771f9d5bd51303de5410ee5afd0a4af9d6572
This commit is contained in:
Siying Dong 2019-05-23 15:53:37 -07:00 committed by Facebook Github Bot
parent 40aa520a51
commit dc30a9b69b
1 changed files with 28 additions and 1 deletions
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29
db/db_iter.h
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@ -21,11 +21,36 @@
namespace rocksdb {
// This file declares the factory functions of DBIter, in its original form
// or a wrapped form with class ArenaWrappedDBIter, which is defined here.
// Class DBIter, which is declared and implemented inside db_iter.cc, is
// a iterator that converts internal keys (yielded by an InternalIterator)
// that were live at the specified sequence number into appropriate user
// keys.
// Each internal key is consist of a user key, a sequence number, and a value
// type. DBIter deals with multiple key versions, tombstones, merge operands,
// etc, and exposes an Iterator.
// For example, DBIter may wrap following InternalIterator:
// user key: AAA value: v3 seqno: 100 type: Put
// user key: AAA value: v2 seqno: 97 type: Put
// user key: AAA value: v1 seqno: 95 type: Put
// user key: BBB value: v1 seqno: 90 type: Put
// user key: BBC value: N/A seqno: 98 type: Delete
// user key: BBC value: v1 seqno: 95 type: Put
// If the snapshot passed in is 102, then the DBIter is expected to
// expose the following iterator:
// key: AAA value: v3
// key: BBB value: v1
// If the snapshot passed in is 96, then it should expose:
// key: AAA value: v1
// key: BBB value: v1
// key: BBC value: v1
//
class Arena;
class DBIter;
// Return a new iterator that converts internal keys (yielded by
// "*internal_iter") that were live at the specified "sequence" number
// "*internal_iter") that were live at the specified `sequence` number
// into appropriate user keys.
extern Iterator* NewDBIterator(
Env* env, const ReadOptions& read_options,
@ -41,6 +66,8 @@ extern Iterator* NewDBIterator(
// a iterator hierarchy whose memory can be allocated inline. In that way,
// accessing the iterator tree can be more cache friendly. It is also faster
// to allocate.
// When using the class's Iterator interface, the behavior is exactly
// the same as the inner DBIter.
class ArenaWrappedDBIter : public Iterator {
public:
virtual ~ArenaWrappedDBIter();