count()很慢怎么办？
2020-03-31sad creeper
count(*) 的实现方式
在不同的 MySQL 引擎中，count(*) 有不同的实现方式。MyISAM 引擎把一个表的总行数存在了磁盘上，因此执行 count(*) 的时候会直接返回这个数，效率很高；而 InnoDB 引擎就麻烦了，它执行 count(*) 的时候，需要把数据一行一行地从引擎里面读出来，然后累积计数。这里需要注意的是，我们在这篇文章里讨论的是没有过滤条件的 count(*)，如果加了 where 条件的话，MyISAM 表也是不能返回得这么快的。
在数据库保存计数
把计数直接放到单独的一张数据表中保存。利用“事务”这个特性，把问题解决掉。
不同的 count 用法
count() 是一个聚合函数，对于返回的结果集，一行行地判断，如果 count 函数的参数不是 NULL，累计值就加 1，否则不加。最后返回累计值。
count(*)、count(主键 id) 和 count(1) 都表示返回满足条件的结果集的总行数；而 count(字段），则表示返回满足条件的数据行里面，参数“字段”不为 NULL 的总个数。
对于 count(主键 id) 来说，InnoDB 引擎会遍历整张表，把每一行的 id 值都取出来，返回给 server 层。server 层拿到 id 后，判断是不可能为空的，就按行累加。
对于 count(1) 来说，InnoDB 引擎遍历整张表，但不取值。server 层对于返回的每一行，放一个数字“1”进去，判断是不可能为空的，按行累加。单看这两个用法的差别的话，你能对比出来，count(1) 执行得要比 count(主键 id) 快。因为从引擎返回 id 会涉及到解析数据行，以及拷贝字段值的操作。
对于 count(字段) 来说：如果这个“字段”是定义为 not null 的话，一行行地从记录里面读出这个字段，判断不能为 null，按行累加；如果这个“字段”定义允许为 null，那么执行的时候，判断到有可能是 null，还要把值取出来再判断一下，不是 null 才累加。也就是前面的第一条原则，server 层要什么字段，InnoDB 就返回什么字段。但是 count(*) 是例外，并不会把全部字段取出来，而是专门做了优化，不取值。count(*) 肯定不是 null，按行累加。
按照效率排序的话，count(字段)<count(主键 id)<count(1)≈count(*)，所以我建议你，尽量使用 count(*)。
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The same factor with HIIT exercises, when you have the power, then do one, if not, then don’t. You wish to keep away from overtraining, but you additionally have to push your self to make vital changes and attain your goals. It is decided by what kind of exercises you propose to do with the battle ropes, your training goals, and what different exercises you're doing. Then, after all, your higher body is working because it does with any waves variation. You are going to be shredding by way of excessive calorie burn and getting a imply pump in your arms, chest, and again. Your core, as all the time, shall be working exhausting in tandem along with your decrease body to stabilize it all. As for length and thickness, typically talking, the finest choice is a 50 foot lengthy (although forty feet will do) 1.5 inch thick battle rope. This actually applies to all health levels and men and women (with exception of massive males and tiny ladies, who might want a thicker or shorter rope, respectively). Right Here are some sample workouts that you can do using the workouts you realized above. We will start with a warm up then get into HIIT workouts, stand-alone exercises, and exercise finishers. By performing both actions, you'll have the ability to strengthen all muscular tissues of the rotator cuff apart from the supraspinatus. One Other exercise that’s nice for building bigger and stronger rear delts is the cable face pull. In Contrast to the rear delt fly, most individuals can move a lot more weight with the cable face pull, permitting you to overload the rear delts to a higher extent. This one is undoubtedly the most effective mass builder out of all of the rear delt isolation exercises. The battle ropes offer a full-body workout, figuring out the arms, legs, and abs. You are pulling one thing towards you, much in the identical means I imagine people of the hunter-gatherer tribes of the stone ages and agrarian societies of antiquity doing everyday. I want that water, animals, vegetation, human over here, so I will tie a rope around it and pull it towards me. Now that you understand I actually have bizarre thoughts flying through my mind, you are able to do it for aesthetic or performance reasons, as an alternative of my early human identification reasons. Shoulder pulleys are an instance of an train your physical therapist could implement that can help you regain passive ROM. As Quickly As passive ROM is restored, you might progress to active-assistive ROM exercises and, finally, active ROM exercises like the ones in this program. The primary perform of the rhomboids is to help within the movement of the scapula. This is an efficient train to do earlier than deadlifts because it enforces correct scapular positioning, which is essential for a correct deadlift type. If you have back or shoulder problems, speak to a healthcare supplier about whether this train is appropriate. While the overhand grip is most well-liked, some trainers suggest utilizing an underhand grip as a variation. By deciding on a weight that lets you execute each motion with control and precision, you possibly can target your shoulder muscles successfully and safely. To execute this train, place yourself facing the cable machine with the handle set at shoulder top. Pull the deal with in course of your face, maintaining a excessive elbow and retracted shoulder blades to target the supposed muscles effectively. Exercises like the cable rear delt row give consideration to the often-neglected posterior deltoids whereas front raises zero in in your anterior delts. The unique ability of cables to pivot and modify allows you to hit each muscle group from an array of instructions that free weights simply can’t replicate. Using cables for lateral raises helps enhance shoulder stability and energy. Clients discover the product useful for physical remedy after shoulder surgical procedure. It Is simple to make use of and provides the results desired by physical therapists. The pulley permits them to perform every day shoulder movement exercises at house. Each push exercise engages your shoulders alongside with your chest and triceps, and every pull train works your shoulders, again, and biceps. The Skin Circle is instead like the Inside Circle except your arms will be transferring clockwise. It’s really a great way to build mobility, stability and endurance on the scapula level. Battle ropes can be a challenging train but there are plenty of modifications that can be made to make it an excellent exercise for novices. Battle ropes make for effective cardio and muscle-building train and would be an excellent option for beginners because of that. Once they get into the underside squat they will then bounce up right into a jump squat while continuing to move the battle ropes. The alternating wave, also recognized as the unilateral waves exercise, is done by swinging every rope one by one. Think About including 4 to 12 sets with a median rep vary of 10 to 20 for extra moderate loads. If you’re on the lookout for easy cable machine rear delt workout routines to add to your exercise routine, the bent-over cable increase is a great staple train to get you started. Performing battle rope workouts may be carried out 2-3 times per week, depending on your targets. Because the ropes are a full-body exercise, I wouldn't advocate doing battle ropes daily as they're too fatiguing. Allow your body time to get well earlier than doing all your subsequent exercise. It’s like doing a run and a weightlifting session on the same time. This will increase your cardiovascular health and your endurance, which is ready to translate to raised performance both with weightlifting and sports activities. If you would possibly be doing a stand-alone battle rope exercise, it can range, however normally no more than half-hour is good. For a stand-alone exercise, you should relaxation about twice as lengthy as your sets. As for warm ups, 5 minutes is loads of time with battle ropes to get the blood pumping. For this train, you may be creating pressure that generates an arc-style wave down the rope towards the anchor point. This is not only a good way to feel the correct positioning for the face pull, it’s a fantastic approach to perform the train. If you get down on the ground to perform the train, and you’ll discover that should you lead with the elbows your arms can’t come anywhere near the bottom. Instead, you’ll need to ensure your hands or at least the nubs of the rope are making contact with the ground with the elbows elevated. But at all times bear in mind to add dumbbell and barbell workout routines along with cables to give the right form and dimension to your shoulders. Do each the workouts collectively with none rest in between each set. Some folks can not use rear delts during fly workouts due to lack of mind and muscle connection. Whether you’re getting battle ropes as a beginner or you’ve been working the health club for years, you can use the battle ropes to create the ideal exercise program. If you may be restricted to your garage, for instance, you have to think about the area, and thus, the size of the battle rope. On that observe, why not head on over to our other article ‘8 Best Front Delt Dumbbell Exercises’ that can help you activate those anterior shoulder muscular tissues. It additionally limits the momentum that might be generated while standing making the incline lateral increase more difficult with less probability of dishonest. The decrease you set your bench, the tougher the movement. Utilizing dumbbells as an alternative of a shoulder press machine or a loaded barbell will put much much less strain on the shoulders and allow for a extra pure arm path. Excessive waves are carried out with an upright torso and create massive waves, placing a lot of work on the upper body, such because the shoulders, arms, traps, lats, and chest. Use the battle ropes to warm up on your energy training or particular battle rope routines but don't go too onerous here; in any other case, you will tire your self out. As you'll find a way to see, you will get super creative with battle ropes. Trapeziuses are muscle tissue that begin at the base of the neck and finish at the upper back. They additionally create a further stimulus for the shoulders, although, and can allow you to to sculpt a extra well-rounded higher physique. Begin by attaching the straight bar to the cable machine and adjusting the cable to its lowest level. Subsequent, follow the steps beneath to finish the cable shoulder exercise. Alternatively, you possibly can add a rope circulate session at the finish of your typical exercise routine. If you’re new to rope move workouts, you’ll probably have to make a DIY move rope, or buy a branded move rope in order to give it a attempt. If this sounds attention-grabbing, you could need to be taught extra about the advantages of rope flow workouts and how one can add them to your workout routine. Cables are a fantastic piece of apparatus to focus on every area of your shoulders. Cable upright rows are a cable shoulder workout that targets the front a half of the shoulder. Upright rows are a great accent train to add to your higher body days. Incorporating a range of units and reps based mostly on every exercise’s issue will aid in progressive overload, resulting in improved strength and definition in the shoulders. It’s necessary to concentrate on maintaining correct form all through all repetitions to make sure optimum muscle contraction and progress. Cable and free weight workouts each have their very own advantages, so it is dependent upon what you’re looking for. Free weights are good at constructing muscle and power, whereas cables offer fixed rigidity and a managed path of motion which makes them good for isolation. Stretching the muscular tissues you have strengthened is important to restore vary of movement and stop damage. After performing strengthening actions, gently stretching your muscular tissues might help prevent ache and preserve their size and adaptability. An train conditioning program will allow you to get back to your common activities and lead a healthier, extra lively life-style following surgery or an damage. Following a fastidiously designed conditioning program may assist you to return to sports and different hobbies that you enjoy. There’s loads of weight with this rope so you'll find a way to maintain things shifting along when you’re figuring out. A rope is a rope however this isn’t your kindergarten leap rope! This is a heavy-duty rope that's thick, heavy, and lengthy and gives you a whole totally different set of resistance. Figuring Out what to do helps you focus in your exercise and gives you plenty of options whenever you need to create a versatile and distinctive workout program. As Soon As the athlete has mastered every of the circuits shown above, I will progress them to those more difficult variations. I will begin the athletes unloaded for each of these circuits and progress each sequence by having the athlete hold 2.5 LB weight plates in every hand. You ought to start with a lighter weight to attain optimal performance. Additionally, your focus ought to be on lifting and decreasing the burden along with your traps alone quite than how a lot you'll have the ability to bounce the load. A controlled movement ensures maximum engagement of the traps and minimizes the danger of damage. Your shrugs should be up and down—no rolling your shoulders. Battle ropes offer advantages for individuals of all fitness levels. They’re simple to arrange, simple to use, require minimal instruction, and will elevate your heart fee with out having to go on these boring cardio machines. Right Here are another fantastic benefits of incorporating battle ropes into your training. Your arms work independently from each other when you’re using the battle ropes so your body starts to compensate and proper any imbalances. Your core is engaged as your body is forced to remain secure and upright with every flick of the wrist. It is a really efficient and practical train for constructing muscle and energy within the posterior deltoids. Additionally, it helps to enhance shoulder stability and posture. Upright row prompts all major shoulder muscular tissues, making it wonderful for higher physique stability and conditioning. It is necessary to do this frequently to enhance your posture and defend yourself from harm.
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