Question

I was browsing through the source code for Mendeley for Android by Martin Eve and saw that Temp tables are created along with the main tables by calling replace on the tables with _id. For example on the Collections table

db.execSQL(COLLECTIONS_CREATE.replace(" (_id", "_TEMP (_id"));

I guess it creates a new temporary table. Any more explanation on this would be great. Further on the data is first inserted to temp tables and later moved to main tables.

I searched through SO and came across What are the use cases for temporary tables in SQL database systems? and saw that temp tables are used for handling complex queries, sorting and for increasing performance. Can anyone explain how it helps in this situation?

Answer 1

Temp tables make things easier for the programmer by letting the programmer break up a single complex query into multiple relatively simpler queries, and also by letting the programmer store results temporarily so they can be consulted multiple times for different purposes during the course of the program without having to be reinstantiated each time. The latter also makes things easier for the computer. The disk subsystem and CPU can take a little rest, so to speak.

An example of the former: let say you wanted to get all records where:

                    the sale was in the eastern division
                    and involved one of the several new gizmos introduced last quarter
                    and occurred during the special 5-day bonanza sale

                    or

                    the sale was made by the boss's daughter
                    who floats from division to division
                    and the sale occurred at any time during the month of May

Your program will then generate an email praising the salesperson for the sale, with a cc to the division manager.

The single query that fetches records that satisfy either of those sets of conditions above might get a little unwieldy–just a little difficult for one’s addled brain or weary eyes to handle after a long day of dealing with the sort of crap one has to deal with in most walks of life. It’s a trivial example, of course; in a production system the conditions are often more complex than those above, involving calculations and tests for null values and all sorts of other tedious things that can cause a query statement to grow long and turn into a ball of tangled yarn.

So if you created a temp table, you could populate the temp table with the rows that satisfy the first set of conditions, and then write the second query that grabs the rows that satisfy the second set of conditions, and insert them into the temp table too, and voilà — your temp table contains all the rows you need to work with, in two baby steps.