[PERFORM] Order By Clause, Slows Query Performance?

Eg. SELECT col1, col2, col3,col10 FROM table1; For above query If I didn't mention ORDER BY clause, then I want to know selected data will appear in which order by a query planner? Because I have huge size table, and when I applied ORDER BY col1, col2..in query the performance is soo bad that

[PERFORM] BitMap Heap Scan & BitMap Index Scan

I would like to know, What is BitMap Heap Scan & BitMap Index Scan?When I use EXPLAIN for query, which has LEFT JOIN with 4 different table thensome time query planner uses Bitmap Heap Scan and some time Bitmap Index Scan? -- View this message in context: http://postgresql.1045698.n5.nabble.com