MySQL Query Fetch Time increases when LEFT JOIN with big table

I am fetching the same amount of data with 2 different queries. However, one of them have a fetching time of ~x130 compared to the other. Being the only difference between the two queries a LEFT JOIN with a big table (4M rows).

Especifically, my problem goes like this:
I have table_a with 200K rows. table_b with 100 rows and table_c with 4M rows. The fields involved are indexed. table_a and table_c have a one to many cardinality.

My query looks something like this:

       SELECT           a.id        FROM           table_a a           LEFT JOIN table_b b ON b.id = a.b_id           LEFT JOIN table_c c ON c.id = b.c_id        GROUP BY a.id; 

MySQL Workbench tells me that this query takes ~4s of Duration and 130 seconds of Fetch Time. However, when I remove the second LEFT JOIN with the big table, the query takes <1s of Duration and <1s Fetch Time.

I clearly understand why the query duration is increased. I am doing a kinda heavy left join. But, my question is: Why the fetching time is so much higher, if the fetched data is the same?

I have already increased innodb_buffer_pool_size with no success.

I am working in MySQL 8.0.19, with innodb as tables engine.

Is there something I am missing here? Thanks in advance for the help!

UPDATE:
Removing the GROUP BY also improves the performance <1s of Duration and ~1s Fetch Time

Is an item that increases spell save DC balanced?

One of my players’ characters likes to use illusion spells in combat. To improve their effectiveness, he is asking me for an item that increases his spell save DC. The Rod of the Pact Keeper provides a spell save DC boost. However, it is locked to Warlocks. I’m considering giving him a similar item without the class restriction.

Example:

Ring of channeling (very rare)
While you wear this ring, the spell save DC of your spells is modified by +2.

Would it be unbalanced to have an item provide just a bump to Spell save DC?

If it is relevant, he is a level 15 Sorcerer and already has Charisma 20.

How does the maximum number of guesses needed to win Mastermind (board game) change as the size of the board increases?

Donald Knuth demonstrated that the codebreaker in the board game Mastermind can solve the pattern in five moves or fewer using the following algorithm:

  1. Create a set S of remaining possibilities (at this point there are 1296). The first guess is aabb.
  2. Remove all possibilities from S that would not give the same score of colored and white pegs if they were the answer.
  3. For each possible guess (not necessarily in S) calculate how many possibilities from S would be eliminated for each possible colored/white score. The score of the guess is the least of such values. Play the guess with the highest score (minimax).
  4. Go back to step 2 until you have got it right.

I am curious: what would be the maximum number of guesses necessary to win a Mastermind-like game with 5 pegs instead of 4? How about 1,000 pegs, or a million?

Magic jar and ability increases

Magic jar p257 of the Player’s Handbook states:

Once you possess a creature’s body, you control it. Your game statistics are replaced by the statistics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can’t use any of its class features.

Since you gain ability increases as part of your class levels, am I correct in assuming you keep them in the new body? Also would I still be able to use a feat like Elven Accuracy, even tho I may no longer be an elf?

Bounded accuracy increases randomness?

I have been reading DnD 5e for a forthcoming campaign and I have a small concern with bounded accuracy: Proficiency bonus growth in a small rate (a level 20 character has only a +6 proficiency bonus whereas in previous DnD incarnations this would be a higher bonus in almost any case). Also magic weapons were nerfed (a Holly Avenger is a +3 sword where in previous version it was a +5 sword).

All in all I think that the designed outcome is that a high level character has a lower bonus to their rolls. I have read the benefits of this and while I agree in some points my concern is that since the bonus have shrinked the d20 has a bigger weight in the action outcome.

Is this a real problem or I am overthinking? It has been addressed by game designers? How do you overcome this in your campaigns?

Have any of the designers of D&D explained why Strength increases Hit Chance?

This is a personal bias, but when I think high strength score, the imagery I get is:

Bill Kazmaier

When it comes to accuracy of a muscle-bound and bulky person, the imagery I get is:

Ali Bobbing and Weaving

Bruce Lee had stated he didn’t want to have overly bulky muscles because it would affect his agility and accuracy. He did believe in weight training to make his muscles more dense, but that mostly was for grip and striking force.1

I started playing with AD&D 2nd Edition, and even back then an 18 strength gave you +1 Hit and +2 to damage.

Have any D&D developers given any reasoning for why the Strength score, specifically, gives a bonus to hit?

As I recall, being big and strong doesn’t actually help you hit a baseball, but may help you in making that ball you hit a home run.

1 Bruce Lee explains this throughout the Tao of Jeet Kune Do.

Does a monster’s Proficiency Bonus increase when its CR increases due to it being in a lair or coven?

Some monsters have a note saying that their CR is increased when in its Lair or Coven. Since the Proficiency bonus is directly tied to the CR/XP-rating, does this mean that it increase when in a Lair or Coven? If it does it would have effect, for example on spellcasting.

Example: The Green Hag (MM p.177). Its ‘normal’ CR is 3 (proficiency bonus +2), but when part of a coven is raised to CR 5 (proficiency bonus +3).

The Hag has Innate Spellcasting (Charisma) and a Charisma Modifier of +2. Her spell save DC is 8+2+2=12. When using CR 5, it would be 8+2+3=13.

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