Super columns suffer from a number of problems, not least of which is that it is necessary for Cassandra to deserialze all of the sub-columns of a super column when querying (even if the result will only return a small subset). As a result, there is a practical limit to the number of sub-columns per … Read more
Yes, index will help you, when using ORDER BY. Because INDEX is a sorted data structure, so the request will be executed faster. Look at this example: table test2 with 3 rows. I used LIMIT after order by to show the difference in execution. DROP TABLE IF EXISTS `test2`; CREATE TABLE `test2` ( `id` int(10) … Read more
For anyone coming here wondering if they need an index_together in addition to unique_together to get the index’s performance benefit, the answer for Postgres is no, they are functionally the same.
First off, depending on the Storage Engine used, you may just not have a choice (InnoDB for example is exclusively using BTREE for its index). Also, BTREE is the default index type for most storage engines. Now… There are cases, when using alternative index types may result in improved performance. There are (relatively rare case) … Read more
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. … Read more
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 … Read more
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 … Read more
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 … Read more
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 … Read more
From the shell: db.test.getIndexes() For shell help you should try: help; db.help(); db.test.help();