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Alter table table1 add index col1_idx (column1);
Even though the table1 has many rows and all of the have non null values for column1, the above query says "0 Rows AFfected". Why could this be?
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Yes you can. It will lock the table you're adding an index to while it's being created. If the table is large, it may take awhile as it has to read each row while building the index.
Adding an index means that the database has to maintain it, that means that it has to be updated, so the more writes you have, the more the index will be updated. Save this answer.
In MySQL, an index can be created on a table when the table is created with CREATE TABLE command. Otherwise, CREATE INDEX enables to add indexes to existing tables. A multiple-column index can be created using multiple columns.
MySQL does not support partial indexes.
That is because the query is not directly affecting any rows as opposed to an update statement where you modify the row. It (very, very, very basically) only changes how the column is stored and sorted.
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