KVDocument里什么情况下会有多个KVIndexDocument ?

看到KVDocument里有个成员变量： std::vector _docs;

看了一些test case和代码，没发现会有多个 _docs的情况

另外单测里，这个case的value.size()为什么是39？：
{
document::KVDocument doc;
RawDocumentParser::Message msg;
string rawData = "userid=100;itemid=1;price=99;title=hello;CMD=add";
msg.data = rawData;
msg.timestamp = 100;
ASSERT_TRUE(parser.parse(msg, doc));
auto kvIndexDoc = doc.begin();
ASSERT_EQ(1u, kvIndexDoc->GetPKeyHash());
ASSERT_FALSE(kvIndexDoc->HasSKey());
ASSERT_EQ(100, doc.GetTimestamp());
ASSERT_EQ(100, kvIndexDoc->GetTimestamp());

ASSERT_EQ(ADD_DOC, kvIndexDoc->GetDocOperateType());
auto value = kvIndexDoc->GetValue();
ASSERT_EQ(39u, value.size());
StringAttributeConvertor convertor;
auto attrMeta = convertor.Decode(value);
ASSERT_EQ(100, *(int64_t*)(attrMeta.data.data() + 1));
ASSERT_EQ(99, *(int64_t*)(attrMeta.data.data() + 1 + sizeof(int64_t)));
std::string expected = "hello";
std::string actual(attrMeta.data.data() + 1 + sizeof(int64_t) + sizeof(int64_t) + 8 + 1);
ASSERT_EQ(expected, actual);
}

想学习下这边的kv、kkv部分，如果能有一些相关的设计文档说明更好，谢谢

Start with the KVDocument and KVIndexDocument declarations, then run the parser test containing the userid/itemid/price/title example. Trace how _docs is populated and how GetValue() is encoded; the documentation is done when it explains when multiple KVIndexDocument objects occur, why value.size() is 39, and how KV and KKV relate.

Written by the indexing model from the issue text.