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Clarify MemTableInfo sequence number docs #14773

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Clarify MemTableInfo sequence number docs #14773

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22 changes: 10 additions & 12 deletions db/memtable.h
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Original file line number Diff line number Diff line change
Expand Up @@ -276,8 +276,8 @@ class ReadOnlyMemTable {

virtual uint64_t GetDataSize() const = 0;

// Returns the sequence number of the first element that was inserted
// into the memtable.
// Returns the smallest sequence number of any element inserted into the
// memtable, or 0 if the memtable is empty.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
virtual SequenceNumber GetFirstSequenceNumber() = 0;
Expand All @@ -287,10 +287,9 @@ class ReadOnlyMemTable {
// operations on the same MemTable (unless this Memtable is immutable).
virtual bool IsEmpty() const = 0;

// Returns the sequence number that is guaranteed to be smaller than or equal
// to the sequence number of any key that could be inserted into this
// memtable. It can then be assumed that any write with a larger(or equal)
// sequence number will be present in this memtable or a later memtable.
// Returns a lower bound on the sequence numbers that may be present in the
// memtable. This can be smaller than the first sequence number, and does not
// necessarily correspond to an entry in the memtable.
//
// If the earliest sequence number could not be determined,
// kMaxSequenceNumber will be returned.
Expand Down Expand Up @@ -783,8 +782,8 @@ class MemTable final : public ReadOnlyMemTable {
return first_seqno_.load(std::memory_order_relaxed);
}

// Returns the sequence number of the first element that was inserted
// into the memtable.
// Sets the smallest sequence number of any element inserted into the
// memtable.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
void SetFirstSequenceNumber(SequenceNumber first_seqno) {
Expand All @@ -796,10 +795,9 @@ class MemTable final : public ReadOnlyMemTable {
return earliest_seqno_.load(std::memory_order_relaxed);
}

// Sets the sequence number that is guaranteed to be smaller than or equal
// to the sequence number of any key that could be inserted into this
// memtable. It can then be assumed that any write with a larger(or equal)
// sequence number will be present in this memtable or a later memtable.
// Sets the lower bound on the sequence numbers that may be present in the
// memtable. This can be smaller than the first sequence number, and does not
// necessarily correspond to an entry in the memtable.
// Used only for MemPurge operation
void SetEarliestSequenceNumber(SequenceNumber earliest_seqno) {
return earliest_seqno_.store(earliest_seqno, std::memory_order_relaxed);
Expand Down
11 changes: 5 additions & 6 deletions include/rocksdb/listener.h
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Original file line number Diff line number Diff line change
Expand Up @@ -499,13 +499,12 @@ struct CompactionJobInfo {
struct MemTableInfo {
// the name of the column family to which memtable belongs
std::string cf_name;
// Sequence number of the first element that was inserted
// into the memtable.
// Smallest sequence number of any element inserted into the memtable, or 0
// if the memtable is empty.
SequenceNumber first_seqno;
// Sequence number that is guaranteed to be smaller than or equal
// to the sequence number of any key that could be inserted into this
// memtable. It can then be assumed that any write with a larger(or equal)
// sequence number will be present in this memtable or a later memtable.
// Lower bound on the sequence numbers that may be present in the memtable.
// This can be smaller than first_seqno, and does not necessarily correspond
// to an entry in the memtable.
SequenceNumber earliest_seqno;
// Total number of entries in memtable
uint64_t num_entries;
Expand Down