facebook#42: Range in form tbl.key > const doesn't work in reverse co…

…lumn family

Summary:
Fix issue facebook#42. Also issue facebook#43: Ranges on t.key > const are not handled
efficiently:

Make ha_rocksdb::index_read_map() do the right thing when invoked with
find_flag=HA_READ_KEY_OR_NEXT, both for forward and reverse column
families.

Detailed explanation of actions is provided in rocksdb-range-access.txt

Differential Revision: https://reviews.facebook.net/D35103

storage/rocksdb/ha_rocksdb.cc

@@ -2696,7 +2696,8 @@ int ha_rocksdb::index_read_map(uchar *buf, const uchar *key,
                                     keypart_map);
 
   if (find_flag == HA_READ_PREFIX_LAST_OR_PREV ||
-      find_flag == HA_READ_PREFIX_LAST)
+      find_flag == HA_READ_PREFIX_LAST ||
+      find_flag == HA_READ_AFTER_KEY)
   {
     /* See below */
     kd->successor(sec_key_packed_tuple, packed_size);
@@ -2715,7 +2716,7 @@ int ha_rocksdb::index_read_map(uchar *buf, const uchar *key,
     greater than the lookup tuple.
   */
   setup_index_scan(kd, &slice, use_all_keys, is_ascending(kd, find_flag));
-  bool move_forward= true;
+  //bool move_forward= true;
 
 
   switch (find_flag) {
@@ -2753,11 +2754,45 @@ int ha_rocksdb::index_read_map(uchar *buf, const uchar *key,
     }
     break;
   }
+  case HA_READ_BEFORE_KEY:
+  {
+    //move_forward= false;
+    /* We want to read the record that's right *before* the given key.  */
+    if (!scan_it->Valid())
+    {
+      /*
+        All the values in the database are smaller than our key. Two cases
+         - our index is the last in db. Its last value is a match
+         - our index has no records (in that case we will get a record from
+           our index and detect it below)
+      */
+      scan_it->SeekToLast();
+    }
+    else
+    {
+      /*
+        RocksDB iterator is positioned at "the first key in the source that
+        at or past target".
+        We need to step one key back, so that we're at the last key that is
+        before the target.
+        If the passed key is greater than the max. value that is found in the
+        table, then iterator is pointing at the *first* record in subsequent
+        table/index.
+      */
+      scan_it->Prev();
+    }
+    /* fall through */
+  }
+  case HA_READ_AFTER_KEY:
   case HA_READ_KEY_OR_NEXT:
   {
     /*
       We are looking for the first record such that
-        index_tuple >= lookup_tuple.
+
+        index_tuple $GT lookup_tuple
+
+      with HA_READ_AFTER_KEY, $GT = '>',
+      with HA_READ_KEY_OR_NEXT, $GT = '>='
     */
     if (key_descr[active_index]->is_reverse_cf)
     {
@@ -2768,8 +2803,9 @@ int ha_rocksdb::index_read_map(uchar *buf, const uchar *key,
       else
       {
         /*
-          When using a full extended key tuple: we should not step back if
-          we've found an exact match.
+          We should step back
+           - when not using full extended key
+           - when using full extended key and when we've got an exact match
         */
         rkey= scan_it->key();
         if (!using_full_key || !kd->value_matches_prefix(rkey, slice))
@@ -2794,61 +2830,6 @@ int ha_rocksdb::index_read_map(uchar *buf, const uchar *key,
     }
     break;
   }
-  case HA_READ_BEFORE_KEY:
-  {
-    move_forward= false;
-    /* We want to read the record that's right *before* the given key.  */
-    if (!scan_it->Valid())
-    {
-      /*
-        All the values in the database are smaller than our key. Two cases
-         - our index is the last in db. Its last value is a match
-         - our index has no records (in that case we will get a record from
-           our index and detect it below)
-      */
-      scan_it->SeekToLast();
-    }
-    else
-    {
-      /*
-        RocksDB iterator is positioned at "the first key in the source that
-        at or past target".
-        We need to step one key back, so that we're at the last key that is
-        before the target.
-        If the passed key is greater than the max. value that is found in the
-        table, then iterator is pointing at the *first* record in subsequent
-        table/index.
-      */
-      scan_it->Prev();
-    }
-    /* fall through */
-  }
-  case HA_READ_AFTER_KEY:
-  {
-    bool in_key;
-    bool have_row;
-    /*
-      Walk forward until we've found a record that is not equal to the lookup
-      tuple, but still belongs to this index.
-    */
-    while ((have_row= scan_it->Valid()))
-    {
-      rkey= scan_it->key();
-      if (!(in_key= kd->covers_key(rkey.data(), rkey.size())) ||
-          kd->cmp_full_keys(rkey.data(), rkey.size(),
-                            slice.data(), slice.size(),
-                            n_used_parts))
-        break;
-
-      if (move_forward)
-        scan_it->Next();
-      else
-        scan_it->Prev();
-    }
-    if (!have_row || !in_key)
-      rc= HA_ERR_END_OF_FILE;
-    break;
-  }
   case HA_READ_KEY_OR_PREV:
   {
     if (!scan_it->Valid())

storage/rocksdb/rocksdb-range-access.txt

@@ -139,3 +139,78 @@ RocksDB calls:
 
   if (it->Valid() && kd->covers_key(..))
     return record.
+
+== HA_READ_AFTER_KEY, forward CF ==
+
+This is finding min(key) such that key > lookup_key.
+
+Suppose lookup_key = kv-bbb
+
+  ( kv )-aaa-pk
+# ( kv )-bbb
+  ( kv )-bbb-pk1  <--- Seek("kv-bbb") will put us here. We need to
+  ( kv )-bbb-pk2       get to the value that is next after 'bbb'.
+  ( kv )-bbb-pk3
+  ( kv )-bbb-pk4
+  ( kv )-bbb-pk5
+  ( kv )-ccc-pkN  <-- That is, we need to be here.
+
+However, we don't know that the next value is kv-ccc. Instead, we seek to the
+first value that strictly greater than 'kv-bbb'. It is Successor(kv-bbb).
+
+It doesn't matter if we're using a full extended key or not.
+
+RocksDB calls:
+
+  Seek(Successor(kv-bbb));
+  if (it->Valid() && kd->covers_key(it.key()))
+    return record;
+
+Note that the code is the same as with HA_READ_KEY_OR_NEXT, except that
+we seek to Successor($lookup_key) instead of $lookup_key itself.
+
+== HA_READ_AFTER_KEY, backward CF ==
+
+Suppose, the lookup key is 'kv-bbb':
+
+  (kv+1)-xxx-pk
+  ( kv )-ccc-pk7
+  ( kv )-ccc-pk6      <-- We need to be here.
+# Successor(kv-bbb)   <-- We get here when we call Seek(Successor(kv-bbb))
+  ( kv )-bbb-pk5          and we will need to call Prev() (*)
+  ( kv )-bbb-pk4
+  ( kv )-bbb-pk3
+  ( kv )-bbb-pk2
+  ( kv )-bbb-pk1
+# ( kv )-bbb          <-- We would get here if we called Seek(kv-bbb).
+  ( kv )-aaa-pk
+
+(*) - unless Successor(kv-bbb)=(kv-ccc), and Seek(kv-ccc) hits the row. In
+that case, we won't need to call Prev().
+
+RocksDB calls:
+
+  Seek(Successor(kv-bbb));
+  if (!it->Valid())
+  {
+    /*
+      We may get EOF if rows with 'kv-bbb' (below the Successor... line in the
+      diagram) do not exist. This doesn't mean that rows with values kv-ccc
+      do not exist.
+    */
+    it->SeekToLast();
+  }
+  else
+  {
+    if (!using_full_key ||
+        !kd->value_matches_prefix(...))
+    {
+      it->Prev();
+    }
+  }
+
+  if (it->Valid() && kd->covers_key(...))
+    return record.
+
+Note that the code is the same as with HA_READ_KEY_OR_NEXT, except that
+we seek to Successor($lookup_key) instead of $lookup_key itself.