一起做个简单的数据库（三）：一个在内存中仅能做追加操作的单表数据库
本系列文章一共13篇，本文为第3篇，请关注公众号，后续文章会陆续发布。系列文章列表：《手把手教你从零开始实现一个数据库系统》《世上最简单的SQL编译器和虚拟机》我们将从数据库的诸多限制开...
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本系列文章一共13篇，本文为第3篇，请关注公众号，后续文章会陆续发布。
系列文章列表：
我们将从数据库的诸多限制开始，比如，这个数据库将会：
-
仅支持两个操作：插入行并打印所有行
-
数据只会存储在内存中（不会在硬盘做持久化存储）
-
支持硬编码（hard coded）单表
我们的硬编码（hard coded）表将会按如下结构存储信息：
| column | type |
|---|---|
| id | integer |
| username | varchar(32) |
| varchar(255) |
这个表的数据模式非常简单，但是它可以支持多种数据类型和不同大小的文本数据类型。
Insert statement如下：
insert 1 cstack foo@bar.com
我们还需要升级prepare_statement用来解析该语句：
if (strncmp(input_buffer->buffer, "insert", 6) == 0) {
statement->type = STATEMENT_INSERT;
+ int args_assigned = sscanf(
+ input_buffer->buffer, "insert %d %s %s", &(statement->row_to_insert.id),
+ statement->row_to_insert.username, statement->row_to_insert.email);
+ if (args_assigned < 3) {
+ return PREPARE_SYNTAX_ERROR;
+ }
return PREPARE_SUCCESS;
}
if (strcmp(input_buffer->buffer, "select") == 0) {
我们将这些解析后的参数存储到语句对象内部的new Row data中：
+#define COLUMN_USERNAME_SIZE 32
+#define COLUMN_EMAIL_SIZE 255
+typedef struct {
+ uint32_t id;
+ char username[COLUMN_USERNAME_SIZE];
+ char email[COLUMN_EMAIL_SIZE];
+} Row;
+
typedef struct {
StatementType type;
+ Row row_to_insert; // only used by insert statement
} Statement;
现在我们需要拷贝这些数据并以某种格式存储在表中。SQLite使用B树进行快速查找，插入和删除。我们将从简单的内容开始。像B树一样，它将rows分组并存储在page中， page不会被排列为树，page会以数组的形式排列。
我的定义如下：
-
page：存储在内存块中的rows
-
每个page存储尽可能多的rows
-
rows被序列化的存储在每一个page的compact representation中
page按需分配
-
指向page的指针数组固定大小
首先，我们定义row的compact representation：
+#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)0)->Attribute)
+
+const uint32_t ID_SIZE = size_of_attribute(Row, id);
+const uint32_t USERNAME_SIZE = size_of_attribute(Row, username);
+const uint32_t EMAIL_SIZE = size_of_attribute(Row, email);
+const uint32_t ID_OFFSET = 0;
+const uint32_t USERNAME_OFFSET = ID_OFFSET + ID_SIZE;
+const uint32_t EMAIL_OFFSET = USERNAME_OFFSET + USERNAME_SIZE;
+const uint32_t ROW_SIZE = ID_SIZE + USERNAME_SIZE + EMAIL_SIZE;
被序列化的row将按如下布局：
| column | size (bytes) | offset |
|---|---|---|
| id | 4 | 0 |
| username | 32 | 4 |
| 255 | 36 | |
| total | 291 |
我们还需要代码来转换compact representation：
+void serialize_row(Row* source, void* destination) {
+ memcpy(destination + ID_OFFSET, &(source->id), ID_SIZE);
+ memcpy(destination + USERNAME_OFFSET, &(source->username), USERNAME_SIZE);
+ memcpy(destination + EMAIL_OFFSET, &(source->email), EMAIL_SIZE);
+}
+
+void deserialize_row(void* source, Row* destination) {
+ memcpy(&(destination->id), source + ID_OFFSET, ID_SIZE);
+ memcpy(&(destination->username), source + USERNAME_OFFSET, USERNAME_SIZE);
+ memcpy(&(destination->email), source + EMAIL_OFFSET, EMAIL_SIZE);
+}
接下来用一个Table结构指向page并跟踪有多少row：
+const uint32_t PAGE_SIZE = 4096;
+#define TABLE_MAX_PAGES 100
+const uint32_t ROWS_PER_PAGE = PAGE_SIZE / ROW_SIZE;
+const uint32_t TABLE_MAX_ROWS = ROWS_PER_PAGE * TABLE_MAX_PAGES;
+
+typedef struct {
+ uint32_t num_rows;
+ void* pages[TABLE_MAX_PAGES];
+} Table;
我将page大小设为4 KB，因为它与大多数计算机体系结构的虚拟内存系统使用的page大小相同。这意味着数据库中的page对应于操作系统使用的page。操作系统将page作为整体单元移入和移出内存，而不是将其拆分。
我将100作为我们可分配的page数量的上限。当我们切换到树形结构时，数据库的最大大小将仅受文件最大大小的限制。（我们仍然会限制内存中page的数量。）
Rows应该不会溢出page的边界。由于内存中的page大概率不会彼此相邻，如果假设正确，读取/写入rows会更加容易。
说到这，下面就是我们如何确定特定row在内存中的读取/写入位置：
+void* row_slot(Table* table, uint32_t row_num) {
+ uint32_t page_num = row_num / ROWS_PER_PAGE;
+ void* page = table->pages[page_num];
+ if (page == NULL) {
+ // Allocate memory only when we try to access page
+ page = table->pages[page_num] = malloc(PAGE_SIZE);
+ }
+ uint32_t row_offset = row_num % ROWS_PER_PAGE;
+ uint32_t byte_offset = row_offset * ROW_SIZE;
+ return page + byte_offset;
+}
现在我们可以用execute_statement从表中进行读写：
-void execute_statement(Statement* statement) {
+ExecuteResult execute_insert(Statement* statement, Table* table) {
+ if (table->num_rows >= TABLE_MAX_ROWS) {
+ return EXECUTE_TABLE_FULL;
+ }
+
+ Row* row_to_insert = &(statement->row_to_insert);
+
+ serialize_row(row_to_insert, row_slot(table, table->num_rows));
+ table->num_rows += 1;
+
+ return EXECUTE_SUCCESS;
+}
+
+ExecuteResult execute_select(Statement* statement, Table* table) {
+ Row row;
+ for (uint32_t i = 0; i < table->num_rows; i++) {
+ deserialize_row(row_slot(table, i), &row);
+ print_row(&row);
+ }
+ return EXECUTE_SUCCESS;
+}
+
+ExecuteResult execute_statement(Statement* statement, Table* table) {
switch (statement->type) {
case (STATEMENT_INSERT):
- printf("This is where we would do an insert.\n");
- break;
+ return execute_insert(statement, table);
case (STATEMENT_SELECT):
- printf("This is where we would do a select.\n");
- break;
+ return execute_select(statement, table);
}
}
最后我们需要初始化表，创建内存释放函数并且处理一些小错误：
+ Table* new_table() {
+ Table* table = malloc(sizeof(Table));
+ table->num_rows = 0;
+ for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
+ table->pages[i] = NULL;
+ }
+ return table;
+}
+
+void free_table(Table* table) {
+ for (int i = 0; table->pages[i]; i++) {
+ free(table->pages[i]);
+ }
+ free(table);
+}
int main(int argc, char* argv[]) {
+ Table* table = new_table();
InputBuffer* input_buffer = new_input_buffer();
while (true) {
print_prompt();
@@ -105,13 +203,22 @@ int main(int argc, char* argv[]) {
switch (prepare_statement(input_buffer, &statement)) {
case (PREPARE_SUCCESS):
break;
+ case (PREPARE_SYNTAX_ERROR):
+ printf("Syntax error. Could not parse statement.\n");
+ continue;
case (PREPARE_UNRECOGNIZED_STATEMENT):
printf("Unrecognized keyword at start of '%s'.\n",
input_buffer->buffer);
continue;
}
- execute_statement(&statement);
- printf("Executed.\n");
+ switch (execute_statement(&statement, table)) {
+ case (EXECUTE_SUCCESS):
+ printf("Executed.\n");
+ break;
+ case (EXECUTE_TABLE_FULL):
+ printf("Error: Table full.\n");
+ break;
+ }
}
}
通过这些更改，我们实际上可以将数据保存在数据库中！
~ ./db
db > insert 1 cstack foo@bar.com
Executed.
db > insert 2 bob bob@example.com
Executed.
db > select
(1, cstack, foo@bar.com)
(2, bob, bob@example.com)
Executed.
db > insert foo bar 1
Syntax error. Could not parse statement.
db > .exit
~
现在，基于如下原因，我们可以进行测试了：
-
我们计划大幅改变存储表的数据结构以增强数据表，并且测试将捕获回归
-
有些极端情况我们并没有测试（比如将数据表写满）
下一章我们回来处理这些问题，本章的代码如下：
@@ -2,6 +2,7 @@
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
+#include <stdint.h>
typedef struct {
char* buffer;
@@ -10,6 +11,105 @@ typedef struct {
} InputBuffer;
+typedef enum { EXECUTE_SUCCESS, EXECUTE_TABLE_FULL } ExecuteResult;
+
+typedef enum {
+ META_COMMAND_SUCCESS,
+ META_COMMAND_UNRECOGNIZED_COMMAND
+} MetaCommandResult;
+
+typedef enum {
+ PREPARE_SUCCESS,
+ PREPARE_SYNTAX_ERROR,
+ PREPARE_UNRECOGNIZED_STATEMENT
+ } PrepareResult;
+
+typedef enum { STATEMENT_INSERT, STATEMENT_SELECT } StatementType;
+
+#define COLUMN_USERNAME_SIZE 32
+#define COLUMN_EMAIL_SIZE 255
+typedef struct {
+ uint32_t id;
+ char username[COLUMN_USERNAME_SIZE];
+ char email[COLUMN_EMAIL_SIZE];
+} Row;
+
+typedef struct {
+ StatementType type;
+ Row row_to_insert; //only used by insert statement
+} Statement;
+
+#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)0)->Attribute)
+
+const uint32_t ID_SIZE = size_of_attribute(Row, id);
+const uint32_t USERNAME_SIZE = size_of_attribute(Row, username);
+const uint32_t EMAIL_SIZE = size_of_attribute(Row, email);
+const uint32_t ID_OFFSET = 0;
+const uint32_t USERNAME_OFFSET = ID_OFFSET + ID_SIZE;
+const uint32_t EMAIL_OFFSET = USERNAME_OFFSET + USERNAME_SIZE;
+const uint32_t ROW_SIZE = ID_SIZE + USERNAME_SIZE + EMAIL_SIZE;
+
+const uint32_t PAGE_SIZE = 4096;
+#define TABLE_MAX_PAGES 100
+const uint32_t ROWS_PER_PAGE = PAGE_SIZE / ROW_SIZE;
+const uint32_t TABLE_MAX_ROWS = ROWS_PER_PAGE * TABLE_MAX_PAGES;
+
+typedef struct {
+ uint32_t num_rows;
+ void* pages[TABLE_MAX_PAGES];
+} Table;
+
+void print_row(Row* row) {
+ printf("(%d, %s, %s)\n", row->id, row->username, row->email);
+}
+
+void serialize_row(Row* source, void* destination) {
+ memcpy(destination + ID_OFFSET, &(source->id), ID_SIZE);
+ memcpy(destination + USERNAME_OFFSET, &(source->username), USERNAME_SIZE);
+ memcpy(destination + EMAIL_OFFSET, &(source->email), EMAIL_SIZE);
+}
+
+void deserialize_row(void *source, Row* destination) {
+ memcpy(&(destination->id), source + ID_OFFSET, ID_SIZE);
+ memcpy(&(destination->username), source + USERNAME_OFFSET, USERNAME_SIZE);
+ memcpy(&(destination->email), source + EMAIL_OFFSET, EMAIL_SIZE);
+}
+
+void* row_slot(Table* table, uint32_t row_num) {
+ uint32_t page_num = row_num / ROWS_PER_PAGE;
+ void *page = table->pages[page_num];
+ if (page == NULL) {
+ // Allocate memory only when we try to access page
+ page = table->pages[page_num] = malloc(PAGE_SIZE);
+ }
+ uint32_t row_offset = row_num % ROWS_PER_PAGE;
+ uint32_t byte_offset = row_offset * ROW_SIZE;
+ return page + byte_offset;
+}
+
+Table* new_table() {
+ Table* table = malloc(sizeof(Table));
+ table->num_rows = 0;
+ for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
+ table->pages[i] = NULL;
+ }
+ return table;
+}
+
+void free_table(Table* table) {
+ for (int i = 0; table->pages[i]; i++) {
+ free(table->pages[i]);
+ }
+ free(table);
+}
+
InputBuffer* new_input_buffer() {
InputBuffer* input_buffer = malloc(sizeof(InputBuffer));
input_buffer->buffer = NULL;
@@ -40,17 +140,105 @@ void close_input_buffer(InputBuffer* input_buffer) {
free(input_buffer);
}
+MetaCommandResult do_meta_command(InputBuffer* input_buffer, Table *table) {
+ if (strcmp(input_buffer->buffer, ".exit") == 0) {
+ close_input_buffer(input_buffer);
+ free_table(table);
+ exit(EXIT_SUCCESS);
+ } else {
+ return META_COMMAND_UNRECOGNIZED_COMMAND;
+ }
+}
+
+PrepareResult prepare_statement(InputBuffer* input_buffer,
+ Statement* statement) {
+ if (strncmp(input_buffer->buffer, "insert", 6) == 0) {
+ statement->type = STATEMENT_INSERT;
+ int args_assigned = sscanf(
+ input_buffer->buffer, "insert %d %s %s", &(statement->row_to_insert.id),
+ statement->row_to_insert.username, statement->row_to_insert.email
+ );
+ if (args_assigned < 3) {
+ return PREPARE_SYNTAX_ERROR;
+ }
+ return PREPARE_SUCCESS;
+ }
+ if (strcmp(input_buffer->buffer, "select") == 0) {
+ statement->type = STATEMENT_SELECT;
+ return PREPARE_SUCCESS;
+ }
+
+ return PREPARE_UNRECOGNIZED_STATEMENT;
+}
+
+ExecuteResult execute_insert(Statement* statement, Table* table) {
+ if (table->num_rows >= TABLE_MAX_ROWS) {
+ return EXECUTE_TABLE_FULL;
+ }
+
+ Row* row_to_insert = &(statement->row_to_insert);
+
+ serialize_row(row_to_insert, row_slot(table, table->num_rows));
+ table->num_rows += 1;
+
+ return EXECUTE_SUCCESS;
+}
+
+ExecuteResult execute_select(Statement* statement, Table* table) {
+ Row row;
+ for (uint32_t i = 0; i < table->num_rows; i++) {
+ deserialize_row(row_slot(table, i), &row);
+ print_row(&row);
+ }
+ return EXECUTE_SUCCESS;
+}
+
+ExecuteResult execute_statement(Statement* statement, Table *table) {
+ switch (statement->type) {
+ case (STATEMENT_INSERT):
+ return execute_insert(statement, table);
+ case (STATEMENT_SELECT):
+ return execute_select(statement, table);
+ }
+}
+
int main(int argc, char* argv[]) {
+ Table* table = new_table();
InputBuffer* input_buffer = new_input_buffer();
while (true) {
print_prompt();
read_input(input_buffer);
- if (strcmp(input_buffer->buffer, ".exit") == 0) {
- close_input_buffer(input_buffer);
- exit(EXIT_SUCCESS);
- } else {
- printf("Unrecognized command '%s'.\n", input_buffer->buffer);
+ if (input_buffer->buffer[0] == '.') {
+ switch (do_meta_command(input_buffer, table)) {
+ case (META_COMMAND_SUCCESS):
+ continue;
+ case (META_COMMAND_UNRECOGNIZED_COMMAND):
+ printf("Unrecognized command '%s'\n", input_buffer->buffer);
+ continue;
+ }
+ }
+
+ Statement statement;
+ switch (prepare_statement(input_buffer, &statement)) {
+ case (PREPARE_SUCCESS):
+ break;
+ case (PREPARE_SYNTAX_ERROR):
+ printf("Syntax error. Could not parse statement.\n");
+ continue;
+ case (PREPARE_UNRECOGNIZED_STATEMENT):
+ printf("Unrecognized keyword at start of '%s'.\n",
+ input_buffer->buffer);
+ continue;
+ }
+
+ switch (execute_statement(&statement, table)) {
+ case (EXECUTE_SUCCESS):
+ printf("Executed.\n");
+ break;
+ case (EXECUTE_TABLE_FULL):
+ printf("Error: Table full.\n");
+ break;
}
}
}
原文链接：https://cstack.github.io/db_tutorial/parts/part3.html
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