implement lower bound for iterators

Summary:
- for `SeekToFirst()`, just convert it to a regular `Seek()` if lower bound is specified
- for operations that iterate backwards over user keys (`SeekForPrev`, `SeekToLast`, `Prev`), change `PrevInternal` to check whether user key went below lower bound every time the user key changes -- same approach we use to ensure we stay within a prefix when `prefix_same_as_start=true`.
Closes https://github.com/facebook/rocksdb/pull/3074

Differential Revision: D6158654

Pulled By: ajkr

fbshipit-source-id: cb0e3a922e2650d2cd4d1c6e1c0f1e8b729ff518

HISTORY.md

@@ -1,6 +1,7 @@
 # Rocksdb Change Log
 ### New Features
 * Upon snapshot release, recompact bottommost files containing deleted/overwritten keys that previously could not be dropped due to the snapshot. This alleviates space-amp caused by long-held snapshots.
+* Support lower bound on iterators specified via `ReadOptions::iterate_lower_bound`.
 
 ## 5.7.4 (08/31/2017)
 No significant changes.

db/db_iter.cc

@@ -117,6 +117,7 @@ class DBIter: public Iterator {
         current_entry_is_merged_(false),
         statistics_(cf_options.statistics),
         version_number_(version_number),
+        iterate_lower_bound_(read_options.iterate_lower_bound),
         iterate_upper_bound_(read_options.iterate_upper_bound),
         prefix_same_as_start_(read_options.prefix_same_as_start),
         pin_thru_lifetime_(read_options.pin_data),
@@ -275,6 +276,7 @@ class DBIter: public Iterator {
   uint64_t max_skippable_internal_keys_;
   uint64_t num_internal_keys_skipped_;
   uint64_t version_number_;
+  const Slice* iterate_lower_bound_;
   const Slice* iterate_upper_bound_;
   IterKey prefix_start_buf_;
   Slice prefix_start_key_;
@@ -686,6 +688,14 @@ void DBIter::PrevInternal() {
       return;
     }
 
+    if (iterate_lower_bound_ != nullptr &&
+        user_comparator_->Compare(saved_key_.GetUserKey(),
+                                  *iterate_lower_bound_) < 0) {
+      // We've iterated earlier than the user-specified lower bound.
+      valid_ = false;
+      return;
+    }
+
     if (FindValueForCurrentKey()) {
       valid_ = true;
       if (!iter_->Valid()) {
@@ -1078,6 +1088,10 @@ void DBIter::SeekToFirst() {
   if (prefix_extractor_ != nullptr) {
     max_skip_ = std::numeric_limits<uint64_t>::max();
   }
+  if (iterate_lower_bound_ != nullptr) {
+    Seek(*iterate_lower_bound_);
+    return;
+  }
   direction_ = kForward;
   ReleaseTempPinnedData();
   ResetInternalKeysSkippedCounter();

db/db_iter_test.cc

@@ -2832,6 +2832,74 @@ TEST_F(DBIteratorTest, SeekPrefixTombstones) {
   ASSERT_EQ(skipped_keys, 0);
 }
 
+TEST_F(DBIteratorTest, SeekToFirstLowerBound) {
+  const int kNumKeys = 3;
+  for (int i = 0; i < kNumKeys + 2; ++i) {
+    // + 2 for two special cases: lower bound before and lower bound after the
+    // internal iterator's keys
+    TestIterator* internal_iter = new TestIterator(BytewiseComparator());
+    for (int j = 1; j <= kNumKeys; ++j) {
+      internal_iter->AddPut(std::to_string(j), "val");
+    }
+    internal_iter->Finish();
+
+    ReadOptions ro;
+    auto lower_bound_str = std::to_string(i);
+    Slice lower_bound(lower_bound_str);
+    ro.iterate_lower_bound = &lower_bound;
+    Options options;
+    std::unique_ptr<Iterator> db_iter(NewDBIterator(
+        env_, ro, ImmutableCFOptions(options), BytewiseComparator(),
+        internal_iter, 10 /* sequence */,
+        options.max_sequential_skip_in_iterations, 0));
+
+    db_iter->SeekToFirst();
+    if (i == kNumKeys + 1) {
+      // lower bound was beyond the last key
+      ASSERT_FALSE(db_iter->Valid());
+    } else {
+      ASSERT_TRUE(db_iter->Valid());
+      int expected;
+      if (i == 0) {
+        // lower bound was before the first key
+        expected = 1;
+      } else {
+        // lower bound was at the ith key
+        expected = i;
+      }
+      ASSERT_EQ(std::to_string(expected), db_iter->key().ToString());
+    }
+  }
+}
+
+TEST_F(DBIteratorTest, PrevLowerBound) {
+  const int kNumKeys = 3;
+  const int kLowerBound = 2;
+  TestIterator* internal_iter = new TestIterator(BytewiseComparator());
+  for (int j = 1; j <= kNumKeys; ++j) {
+    internal_iter->AddPut(std::to_string(j), "val");
+  }
+  internal_iter->Finish();
+
+  ReadOptions ro;
+  auto lower_bound_str = std::to_string(kLowerBound);
+  Slice lower_bound(lower_bound_str);
+  ro.iterate_lower_bound = &lower_bound;
+  Options options;
+  std::unique_ptr<Iterator> db_iter(NewDBIterator(
+      env_, ro, ImmutableCFOptions(options), BytewiseComparator(),
+      internal_iter, 10 /* sequence */,
+      options.max_sequential_skip_in_iterations, 0));
+
+  db_iter->SeekToLast();
+  for (int i = kNumKeys; i >= kLowerBound; --i) {
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(std::to_string(i), db_iter->key().ToString());
+    db_iter->Prev();
+  }
+  ASSERT_FALSE(db_iter->Valid());
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {

include/rocksdb/options.h

@@ -961,14 +961,24 @@ struct ReadOptions {
   // Default: nullptr
   const Snapshot* snapshot;
 
+  // `iterate_lower_bound` defines the smallest key at which the backward
+  // iterator can return an entry. Once the bound is passed, Valid() will be
+  // false. `iterate_lower_bound` is inclusive ie the bound value is a valid
+  // entry.
+  //
+  // If prefix_extractor is not null, the Seek target and `iterate_lower_bound`
+  // need to have the same prefix. This is because ordering is not guaranteed
+  // outside of prefix domain.
+  //
+  // Default: nullptr
+  const Slice* iterate_lower_bound;
+
   // "iterate_upper_bound" defines the extent upto which the forward iterator
   // can returns entries. Once the bound is reached, Valid() will be false.
   // "iterate_upper_bound" is exclusive ie the bound value is
   // not a valid entry.  If iterator_extractor is not null, the Seek target
   // and iterator_upper_bound need to have the same prefix.
   // This is because ordering is not guaranteed outside of prefix domain.
-  // There is no lower bound on the iterator. If needed, that can be easily
-  // implemented.
   //
   // Default: nullptr
   const Slice* iterate_upper_bound;

options/options.cc

@@ -605,6 +605,7 @@ DBOptions* DBOptions::IncreaseParallelism(int total_threads) {
 
 ReadOptions::ReadOptions()
     : snapshot(nullptr),
+      iterate_lower_bound(nullptr),
       iterate_upper_bound(nullptr),
       readahead_size(0),
       max_skippable_internal_keys(0),
@@ -621,6 +622,7 @@ ReadOptions::ReadOptions()
 
 ReadOptions::ReadOptions(bool cksum, bool cache)
     : snapshot(nullptr),
+      iterate_lower_bound(nullptr),
       iterate_upper_bound(nullptr),
       readahead_size(0),
       max_skippable_internal_keys(0),