I have a sqlite database with about 3 GB of data, most of which is stored in a data table with about 75 million records, having three columns (EntryId INTEGER, PropertyId INTEGER, Value NUMERIC) and PRIMARY KEY(EntryId, PropertyId).
This table is not indexed, to allow faster updates. The problem is that it takes over an hour to access all Values, for a specified PropertyId, when the value is obtained for each EntryId separately (using "SELECT Value FROM Data WHERE PropertyId=? AND EntryId=?", bound to the specified PropertyId and EntryId) and the EntryId values for successive database queries are in essentially random order (taken from an external list of entries that has been sorted by property values). This same query (getting the property value for each EntryId, separately) only takes about 7 minutes when the EntryId values for successive database queries are in the same ascending order as the data orginally inserted into the table. I assume that this has to do with better pager caching of successive records in the database, whereas random access may re-read the same page multiple times (due to the limited cache). My question is whether it should be faster to A) create an index for the table before the query, query the value (for the specified PropertyId) for each EntryId (in essentially random order, from external list of entries), and delete the index after the queries (for each EntryId) are done or B) issue a single "SELECT EntryId, Value FROM Data WHERE PropertyId=?" query (bound to the specified PropertyId) and step through the results, using something like a hash table lookup to map the EntryId values (returned from the query) back to an index into the external list of entries. The values extracted from the database are to be copied into an entry property data structure, having the same order as the external list of entries.
