… single index – there are more complex rules of how indexes will be used if you look at multiple indexes usage with “index merge” Using Index… couple of columns index can be simply much smaller than the data which is one of the reason covering indexes help to speed up queries even if data is in memory. If MySQL is only reading index and not…
Post: Explaining Indexes with a Library Metaphor
August 30, 2011 By 1 Comment
… title in the library. Not using an index If you are not using the index cards, you would have to go shelf… on the hard disk when its not using an index. Using an index You are interested in a book by J.R… saved yourself from walking over to the shelves. Using a covering index You are interested to know how many pages a certain…
Post: Redundant index is not always bad
August 28, 2007 By 7 Comments
… year ago Peter wrote about redundant indexes and mentioned sometimes it is good to leave two indexes, even one is first part… technique we use for such queries is covering index, which store all needed columns in the index, and there is no need to… – 470sec (Here I limited available memory to fit only one index, that shows how can degrade performance if we balance on…
Post: Multi Range Read (MRR) in MySQL 5.6 and MariaDB 5.5
March 21, 2012 By 10 Comments
… traditional secondary index lookups, if the columns that are being fetched do not belong to the secondary index definition (and hence covering index optimization… traditional index lookup (for non-index-only columns) involves, reading an index record, and then using the PK column value in the index record…
Comment: A rule of thumb for choosing column order in indexes
June 7, 2009 By 15 Comments
… sense to consider is covering index potential in particular when trying to target several query patterns with single indexes. Covering index slows down queries which can’t use it as covering index because it is longer but speeds up queries which can use covering index…
Comment: Duplicate indexes and redundant indexes
September 22, 2006 By 38 Comments
… a longer non-unique and non-covering index could give worse performance over a shorter unique index that is the prefix of the… would not have been wasted if the longer index would’ve been a covering index?) I think that is an intersting case too… it kind of approaches Oracle’s index organized tables (IOT’s), which are essentially a big covering index instead of a proper table…
Comment: Full table scan vs full index scan performance
November 23, 2012 By 15 Comments
… not using an index‘) does not distinguish covering and non-covering indexes. It could make sense to log those using a non-covering index and not log those using a covering index. At…
Post: Is it query which needs to be optimized ?
March 6, 2007 By 9 Comments
… is not too large using covering index ie (STATE_ID,CITY_ID,GENDER) is great because scanning index is frequently makes COUNT 5… two queries, because it will not be able to use covering index to compute the rows. If the number of combinations is… efficient – make sure all where clauses are using indexes, try to have queries index covered and avoid joins as much as possible. ORDER…
Comment: Full table scan vs full index scan performance
November 26, 2012 By 15 Comments
…, being able to distinguish full index scans using a covering index and full index scans not using a covering index could be nice. I first thought you wanted to log ALL queries that are non covered by an index. Regarding…
Post: How number of columns affects performance ?
September 28, 2009 By 12 Comments
… common workaround working with such wide tables is to use covering indexes. I added one to t99v1 table and repeated the query… of transaction control overhead attached to them. Also note the covering index scan speed is very similar to full table scan speed… expected as table data is stored in BTREE index very similarly to how indexes are stored. Summary: Beware of dynamic row format…
