This is a great article!
You mention “Use multi-column indexes sparingly“, which is generally good advice, however I would qualify that by saying avoid redundant indexes. It’s important to know that multicolumn (btree) indexes have a specific ordering to the columns. If you don’t filter or join a table using one or more of the columns in that order then it can’t use the index.
For example an index on columns a,b. If you only filter on column b then this index can’t be used. It can be used if you filter by just a, or both a and b. Following on from that, if you frequently filter by both a and b then this is a good index to have. If you also have both a single column index on a, and a multicolumn index on a,b then in that specific case the single column index is the redundant one.
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