> That is, I am not proficient with relational databases, and I can handwave why an SDD is faster, and why data may disappear from an in-memory DB.
Because you know that information for other reasons as most people would. Just because the information is gained for other reasons does not make it irrelevant when using a database though.
> This isn't true. SQLite does not use fixed size records.
It's actually true of most/all modern databases these days. The point isn't knowing the exact structure the database uses to store it's information (even though it can be useful) but knowing how efficiently it can find the information for any given request. Knowing when a database is doing an index lookup or a full table scan is very important and I wouldn't consider someone that can't make a reasonable guess to be proficient in sql. Many of these details are even exposed in the sql, when you create an index and decide if it's clustered or non-clustered your giving the database specific directions about how the data will be physically stored.
The fact that you need to know anything about how they do their work internally to be reasonably competent at using them makes them a leaky abstraction.
SQL leaks for complex queries and schemas if performance needs to be optimized. I argue virtually all abstractions leak heavily when performance is considered, some more than others. SQL leaks relatively little in comparison to some other technologies IME.
Also, SQL has well-established processes and formalisms to design schemas which generally result in solid performance by themselves. That's what RDBMS are around for, after all: enabling efficient and consistent record-oriented data manipulation. This is quite difficult to do correctly in reality; for example, if you write your own transaction mechanism for disk/solid-state storage, you are going to do it wrong. This is genuinely difficult stuff.
There is a ton of internals that SQL abstracts so well that very few DB programmers know or (have to) care about them. Things like commit and rollback protocols, checkpointing, on-disk layouts, I/O scheduling, page allocation strategies, caching etc.
Because you know that information for other reasons as most people would. Just because the information is gained for other reasons does not make it irrelevant when using a database though.
> This isn't true. SQLite does not use fixed size records.
It's actually true of most/all modern databases these days. The point isn't knowing the exact structure the database uses to store it's information (even though it can be useful) but knowing how efficiently it can find the information for any given request. Knowing when a database is doing an index lookup or a full table scan is very important and I wouldn't consider someone that can't make a reasonable guess to be proficient in sql. Many of these details are even exposed in the sql, when you create an index and decide if it's clustered or non-clustered your giving the database specific directions about how the data will be physically stored.
The fact that you need to know anything about how they do their work internally to be reasonably competent at using them makes them a leaky abstraction.