#最近读的东西

#1

Mysql 官方文档

With ON DUPLICATE KEY UPDATE, the affected-rows value per row is 1 if the row is inserted as a new row, 2 if an existing row is updated, and 0 if an existing row is set to its current values. If you specify the CLIENT_FOUND_ROWS flag to the mysql_real_connect() C API function when connecting to mysqld, the affected-rows value is 1 (not 0) if an existing row is set to its current values.

在用upsert语句时，标准的返回值是 1 插入、2 更新、0 无影响。

In assignment value expressions in the ON DUPLICATE KEY UPDATE clause, you can use the VALUES(col_name) function to refer to column values from the INSERT portion of the INSERT … ON DUPLICATE KEY UPDATE statement. In other words, VALUES(col_name) in the ON DUPLICATE KEY UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred.

VALUES() 在上述语句的后面的时候，语法从来没见过，是存在歧义的，是代指数据库里的值还是插入语句中的值。

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INSERT INTO t1 (a,b,c) VALUES (1,2,3),(4,5,6)
  ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

-- 等价于：

INSERT INTO t1 (a,b,c) VALUES (1,2,3)
  ON DUPLICATE KEY UPDATE c=3;
INSERT INTO t1 (a,b,c) VALUES (4,5,6)
  ON DUPLICATE KEY UPDATE c=9;

An INSERT … ON DUPLICATE KEY UPDATE statement against a table having more than one unique or primary key is also marked as unsafe. (Bug #11765650, Bug #58637)

当一个表中有多个唯一索引时，同时使用upsert的结果是不确定的，只会更新随机一个。

#2

在开发时经常会出现 int 与 string 相互转换的场景，所以从int64转换成interface{}的时候，到底发生了什么 👆

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package main

import (
	"strconv"
	"testing"
)

func BenchmarkInterfaceConversion(b *testing.B) {
	num := int64(123)
	var result int64

	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		interfaceNum := interface{}(num)
		result = interfaceNum.(int64)
	}

	_ = result
}

func BenchmarkStrconvConversion(b *testing.B) {
	num := int64(123)
	var result int64

	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		str := strconv.FormatInt(num, 10)
		result, _ = strconv.ParseInt(str, 10, 64)
	}

	_ = result
}
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 [2023/11/03 13:11:15] mbp ➜  ~/Downloads/workspace/ct  go test -bench=. conv_test.go
goos: darwin
goarch: amd64
cpu: Intel(R) Core(TM) i5-1038NG7 CPU @ 2.00GHz
BenchmarkInterfaceConversion-8   	1000000000	         0.4254 ns/op
BenchmarkStrconvConversion-8     	23592367	        52.78 ns/op
PASS
ok  	command-line-arguments	2.849s

#3

为什么对 map 的访问，接了一个参数就不会 panic，还没找到资料

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package main

import (
	"fmt"
	"testing"
)

func TestHelloWorld(t *testing.T) {
	var m map[string]string
	v, ok := m["o"]
	fmt.Printf("value:%+v ok:%+v\n", v, ok)
}
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 [2023/11/03 13:21:49] mbp ➜  ~/Downloads/workspace/ct  go test -v -run . map_test.go
=== RUN   TestHelloWorld
value: ok:false
--- PASS: TestHelloWorld (0.00s)
PASS
ok  	command-line-arguments	0.290s