Index (Only) Scan –> Bitmap Index Scan –> Sequential Scan For few rows it pays to run an index scan. If enough data pages are visible to all (= vacuumed enough, and not too much concurrent write load) and the index can provide all column values needed, then a faster index only scan is used. …
Regardless of how many indices have you created on relation, only one of them will be used in a certain query (which one depends on query, statistics etc). So in your case you wouldn’t get a cumulative advantage from creating two single column indices. To get most performance from index I would suggest to use …
PostgreSQL will use an index only if it thinks it will be cheaper that way. An index on a boolean column, which can only take two possible values, will almost never be used, because it is cheaper to sequentially read the whole table than to use random I/O on the index and the table if …
Yes, MySQL can use multiple index for a single query. The optimizer will determine which indexes will benefit the query. You can use EXPLAIN to obtain information about how MySQL executes a statement. You can add or ignore indexes using hints like so: SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX FOR ORDER BY …
For PostgreSQL, rename_column is implemented as a simple ALTER TABLE … RENAME COLUMN … and that does preserve the indexes. The MySQL versions (both of them) do an ALTER TABLE … CHANGE … and that also preserves the indexes. The SQLite version appears to copy the entire table (with indexes), drop the old one, and …
From the shell: db.test.getIndexes() For shell help you should try: help; db.help(); db.test.help();
Why is it needed? When data is stored on disk-based storage devices, it is stored as blocks of data. These blocks are accessed in their entirety, making them the atomic disk access operation. Disk blocks are structured in much the same way as linked lists; both contain a section for data, a pointer to the …