Powerful ad on anti-terrorism on the eve of Ramadan..!!!

Nested Loop Natural Join

When the optimizer tries to finalize the execution plan of a query, it considers a lot of items. It must take the interrelated decisions based on those items. Most important of those items are, Ø Access path Ø Join Order Ø Join Operation Access path tells how the required data is going to be retrieved from a table. So, this tells nothing but which index scan is imposed on that table like index range, index skip scan and so on. Join Order means, to execute a query that joins more than two tables, Oracle joins two of the tables, and then joins the resulting row source to the next table. This process is continued until all tables are joined into the result. It means oracle can join only two tables at most in a time though more than two tables are referred in a query. Oracle always tries to join the small tables first and then joins with the large tables. The reason behind is, it always tries to lower the number of resultant records formed while in the process of jo...

Bitmap index

Bitmap index is one of the index types that is supported by oracle. Unlike B*tree index, this is very compressed index. It means if I create a bitmap index on a column, then the generated index table will be smaller when compared the index table which is generated by the binary index on the same column. The reason is, in bitmap index, ROWIDs won’t be stored in the index table, instead index values will be mapped against bitmaps. Bitmap is nothing but it stores either of this values, 1(Match) or 0(No Match). For each distinct values in the column, a separate index record will be created. Unlike b*tree index, bitmap index can accept NULL and it creates separate index entry for this. Bitmap index will work better if number of distinct values are relatively less when compared to the total number of records. Why is it referred as “relatively less”? this index will work fine if table of 1,000,000,000 records contains 5000 distinct values and if table of 1,000 records contains 5 distinct va...

IOT – Index Organized Table

Oracle supports these 3 different types of tables, 1. Heap Table (nothing but Data Table) 2. IOT 3. Cluster Table (using Cluster Index) First type, heap table is everyone aware of. Heap Table is of rows and columns and value is stored in each cells (each cell è intersection of each rows and columns). In Oracle, all these tables containing data are stored in the datafiles which would be in HardDisk. In datafile, fundamental storage unit is block (in other databases, this might be “Pages”). Logical representation of the datafile, the block, the data will be like this, “ROWID” is the memory address assigned to the each record getting stored in a block of a datafile. ROWID can be decoded to get the actual block number and the datafile name on which the particular record is stored. . If Suppose say, datafile is of 3MB. If size of single block is say 1MB, then 3 blocks would be in this single data file. Conside...

Playing with Internet

Search This Blog