![]() Tip: The last couple of slides in the video have information that can help.1 Design DB schema and generate SQL code with dbdiagram.io 2 Install & use Docker + Postgres + TablePlus to create DB schema. The column or columns you’d put in the index includes.The column or columns you’d put in the index key.Take some time, sketch it out, see if you can, and think of how it does, and feel free to go ahead and test this out in the WideWorldImporters database yourself, too, before you check your answer in the next video. Given this information, can you design an index that will create two different roads so that these queries just pass one another by and they do not collide in an index? Query on the right is the update query who wants the key lock on the primary key of state provinces. It wants a key lock on the primary key of countries. Is there a way I can get one of these queries onto a different road so that they just pass one another by on roads that are near one another but not the exact same index or the exact same road? Indexes, you know how I said it’s like two queries on a narrow road? My question is always this… ![]() When I am looking at these, and the first thing I do is just try to step through it and understand because just taking in all the information is somewhat time consuming, so hover around, look at stuff, make some notes and figure out what it all means.īut then once I have the information, I try to narrow down what the conflict is and I look at the indexes on the table first. It doesn’t even want a lock anymore because it’s gone.Īnd our query on the right who does the updates is the victor and proceeds. It was killed off, so it released all of its locks. It comes in and looks at them and says, who looks like they should be voted off the island? (I am a Survivor fan.)Īnd in this case, this select query, it was declared the victim. That’s what the deadlock monitor steps in and does! The only way that they can release locks is if something comes in and kills them off entirely and they release their locks forever until they come back and retry. They do not have a mechanism for one of them to temporarily back up and let the other one pass. The way that locking works in SQL Server, because these happen to start and collide at the same time, it’s like they’re on a really narrow road and they just can’t get past one another. The query on the right who’s doing the updates is saying you are blocking my key lock on state provinces, whereas the query on the left is like, well, you’re blocking my key lock on countries. I want, I am requesting a key lock on countries. The other line to the top box says Request Mode. So this one has the key lock on the primary key of the state provinces table. You know that box is the state provinces box. If we go over to the query on the left, the one who has the big old X on it, that’s our victim, so sad! The box at the bottom has an arrow pointing at him, and it says Owner Mode. I want a lock on the StateProvinces table for the query who’s doing the updates So I am requesting, I want a key lock on the primary key of the state provinces table. ![]() That one is, if we look inside the box, Application.StateProvinces. ![]() The arrow at the bottom is pointing to the other box, saying Request Mode: X. I know it’s a key lock because at the top of the box it says key lock. It is an owner, the key lock on the primary key of Application.Countries. That line from the box and the contents in the box itself, it’s saying this query has a key lock. The index name is PK_Application_Countries. When we look over at that circle on the right, the query that’s doing the updates, see the lines coming into it from the boxes? The one that saws Owner Mode: X, that table under the box is the Application.Countries table.Īnd it’s got an index name on there as well. The circles on the graph represent the queries The circle on the right of the graph is that transaction that ends in updating state provinces. It does the joins between cities, states, and countries. Our circle on the left of that deadlock graph is the select query. It’s the exact same thing, just shown in a little tool tip. Again, it’s no more complete than what we saw in the XML. When we hover over those processes, we see that inputbuff. Just like we saw in Management Studio, this is a screenshot of the very same thing. I’ll show you how to interpret this weird little map.Īt the end of the video you get a challenge: can you design an index that will prevent the deadlock from happening again? Transcript This is our frenemy, the deadlock graph Now we’ll step through the deadlock graph
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