Add podcastId index to podcastEpisodes

[drwggm]

I have roughly 40k podcast episodes on around 200 podcasts, and performance was becoming an issue. I noticed the only index on podcastEpisodes was createdAt. Since joins are frequently done between podcasts and podcastEpisodes this will slow things down quite a bit without an index. There is a foreign key constraint, but AFAIK sqlite3 does not create an index for FKs automatically.

This change adds an index on podcastEpisodes.podcastId. It improves performance (loading time) for me on the /library/......, /library/.../bookshelf, and /library/...../podcast/latest webpages

I am not a JS dev. I looked through the migration code, and it seems like the addition of an index should migrate, but I am unsure.

Feel free to ask questions, or I'm on discord too (mggw)

[nichwall]

Would you be able to send me a copy of your database? I was working on putting together a migration with several different new indexes, but I don't have too many podcasts (only about 600 episodes total) for testing various combinations of indexes.

I can just include the createdAt and podcastId but want to try some more.

[advplyr]

We decided it was better to modify indexes in a migration and this was done here #3536