Background

我在业余时间自学数据库，试图通过实现一个入门知识来学习.

您必须实现的第一件事是底层数据格式和存储机制.

在数据库中，有一种称为"Slotted Page"的 struct ，如下所示:

+-----------------------------------------------------------+
| +----------------------+  +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ |
| | HEADER               |  | | | | | | | | | | | | | | | | |
| |                      |  | | | | | | | | | | | | | | | | |
| +----------------------+  +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ |
|                                     SLOT ARRAY            |
|                                                           |
|                                                           |
|                                                           |
|                 +--------------------+ +----------------+ |
|                 |  TUPLE #4          | |  TUPLE #3      | |
|                 |                    | |                | |
|                 +--------------------+ +----------------+ |
|         +--------------------------+ +------------------+ |
|         |  TUPLE #2                | |  TUPLE #1        | |
|         |                          | |                  | |
|         +--------------------------+ +------------------+ |
+-----------------------------------------------------------+

页面数据通过二进制序列化存储到文件.槽是最简单的部分，其中的定义可能如下所示:

struct Slot {
  uint32_t offset;
  uint32_t length;
}

在C++中，读/写过程可能是std::memcpy

// Ignoring offset of header size in below
void write_to_buffer(char *buffer, Slot& slot, uint32_t slot_idx) {
    memcpy(buffer + sizeof(Slot) * slot_idx, &slot.offset, sizeof(uint32_t));
    memcpy(buffer + sizeof(Slot) * slot_idx + sizeof(uint32_t), &slot.length, sizeof(uint32_t));
}

void read_from_buffer(char *buffer, Slot& slot, uint32_t slot_idx) {
    memcpy(&slot.offset, buffer + sizeof(Slot) * slot_idx, sizeof(uint32_t));
    memcpy(&slot.length, buffer + sizeof(Slot) * slot_idx + sizeof(Slot), sizeof(uint32_t));
}

在Java中，据我所知，您可以做两件事中的任何一件:

1. 字节缓冲区

record Slot(int offset, int length) {
    void write(字节缓冲区 buffer) {
        buffer.putInt(offset).putInt(length);
    }
    
    static Slot read(字节缓冲区 buffer) {
        return new Slot(buffer.getInt(), buffer.getInt());
    }
}

2. 新的 foreign 记忆material

record Slot(int offset, int length) {
    public static MemoryLayout LAYOUT = MemoryLayout.structLayout(
            ValueLayout.JAVA_INT.withName("offset"),
            ValueLayout.JAVA_INT.withName("length"));

    public static TupleSlot from(MemorySegment memory) {
        return new TupleSlot(
                memory.get(ValueLayout.JAVA_INT, 0),
                memory.get(ValueLayout.JAVA_INT, Integer.BYTES));
    }

    public void to(MemorySegment memory) {
        memory.set(ValueLayout.JAVA_INT, 0, offset);
        memory.set(ValueLayout.JAVA_INT, Integer.BYTES, length);
    }
}

它们之间的性能差异会是什么？

如果字节缓冲区 API可以忽略不计，我更喜欢它.