Are these WordPress tables safe to add indexes to?

These tables aren’t indexed

These tables are indexed

For example, the post meta table has these indexes:

https://codex.wordpress.org/Database_Description#Indexes_5

You’re more likely to cause issues or reduce performance through bad indexing by adding new indexes. WP already specifies indexes for tables

“dbmain” “wp_mf_post_meta” “post_id” “Not indexed”
“dbmain” “wp_mf_custom_fields” “custom_group_id” “Not indexed”

Those are not WordPress tables, you need to ask the plugin authors.

What I think has happened here is that you’ve used a mystery tool that has given you output that has been misinterpreted.

Notice how it does not say “No Index”, it says Not indexed. It never claims an index is missing. Also notice that wp_comments is listed multiple times. It’s more likely that it’s showing you indexes that already exist, not tables.

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