构建一个简单的数据库系列（二）世界上最简单的sql解析器和虚拟机

英文原链接：https://cstack.github.io/db_tutorial/parts/part2.html

The “front-end” of sqlite is a SQL compiler that parses a string and outputs an internal representation called bytecode.

This bytecode is passed to the virtual machine, which executes it.

sqlite的前端时一个sql编译器，用来解析sql串，然后输出内部编码，称之为bytecode。

bytecode下一步传递给virtual machine进行执行。

本节的目标先识别2个关键字：select,insert..

新增3个函数：

do_meta_command 元数据处理 ，主要处理以.开头的命令，不同于普通的sql。

prepare_statement sql解析成内部编码

execute_statement 执行内部编码

db > select * from t1 where f1=1;
This is where we would do a select.
Executed.
db > DELETE FROM  T1;
Unrecognized keyword at start of 'DELETE FROM  T1;'.
db > INSERT into t1 (f1) values(1);
This is where we would do an insert.
Executed.
db > .exit

Process finished with exit code 0

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* 定义 元数据操作结果*/
enum MetaCommandResult_t {
    META_COMMAND_SUCCESS,
    META_COMMAND_UNRECOGNIZED_COMMAND
};
typedef enum MetaCommandResult_t MetaCommandResult;

/* sql解析结果 */
enum PrepareResult_t {
    PREPARE_SUCCESS,
    PREPARE_UNRECOGNIZED_STATEMENT
};
typedef enum PrepareResult_t PrepareResult;

/* sql type*/
enum StatementType_t{
    STATEMENT_INSERT,
    STATEMENT_SELECT
};
typedef enum StatementType_t StatementType;

struct Statement_t {
    StatementType type;
};
typedef struct Statement_t Statement;

/* 接收输入*/
struct InputBuffer_t {
    char* buffer;
    size_t buffer_length;
    ssize_t input_length;
};
typedef struct InputBuffer_t InputBuffer;

/* 初始化buffer */
InputBuffer* new_input_buffer() {
    InputBuffer* input_buffer = malloc(sizeof(InputBuffer));
    input_buffer->buffer = NULL;
    input_buffer->buffer_length = 0;
    input_buffer->input_length = 0;

    return input_buffer;
}

void print_prompt() { printf("db > "); }

/* 按行从标准输入读取 */
void read_input(InputBuffer* input_buffer) {
    ssize_t bytes_read =
            getline(&(input_buffer->buffer), &(input_buffer->buffer_length), stdin);

    if (bytes_read <= 0) {
        printf("Error reading input\n");
        exit(EXIT_FAILURE);
    }

    // Ignore trailing newline
    input_buffer->input_length = bytes_read - 1;
    input_buffer->buffer[bytes_read - 1] = 0;
}

/* 元数据命令处理 */
MetaCommandResult do_meta_command(InputBuffer* input_buffer) {
    if (strcmp(input_buffer->buffer, ".exit") == 0) {
        exit(EXIT_SUCCESS);
    } else {
        return META_COMMAND_UNRECOGNIZED_COMMAND;
    }
}

/* sql解析 */
PrepareResult prepare_statement(InputBuffer* input_buffer,
                                Statement* statement) {
    if (strncasecmp(input_buffer->buffer, "insert", 6) == 0) {
        statement->type = STATEMENT_INSERT;
        return PREPARE_SUCCESS;
    }
    if (strncasecmp(input_buffer->buffer, "select", 6) == 0) {
        statement->type = STATEMENT_SELECT;
        return PREPARE_SUCCESS;
    }

    return PREPARE_UNRECOGNIZED_STATEMENT;
}

/* sql执行*/
void execute_statement(Statement* statement) {
    switch (statement->type) {
        case (STATEMENT_INSERT):
            printf("This is where we would do an insert.\n");
            break;
        case (STATEMENT_SELECT):
            printf("This is where we would do a select.\n");
            break;
    }
}


/* 主函数*/
int main(int argc, char* argv[])
{
    InputBuffer *input_buffer = new_input_buffer();
    while (true) {
        print_prompt();
        read_input(input_buffer);

        if (input_buffer->buffer[0] == '.') {
            switch (do_meta_command(input_buffer)) {
                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_UNRECOGNIZED_STATEMENT):
                printf("Unrecognized keyword at start of '%s'.\n",
                       input_buffer->buffer);
                continue;
        }

        execute_statement(&statement);
        printf("Executed.\n");
    }
}