A full table scan is a scan made on a database where each row of the table is read in a sequential order and the columns encountered are checked for the validity of a condition. Full table scans are usually the slowest method of scanning a table due to the heavy amount of I/O reads required from the disk which consists of multiple seeks as well as costly disk to memory transfers.
Overview
In a database, a query that is not indexed results in a full table scan, where the database processes each record of the table to find all records meeting the given requirements. Even if the query selects just a few rows from the table, all rows in the entire table will be examined. This usually results in suboptimal performance but may be acceptable with very small tables or when the overhead of keeping indexes up to date is high.
The most important factor in choosing depends on speed. This means that a full table scan should be used when it is the fastest and cannot use a different access path. Several full table scan examples are as follows.
No index
The optimizer must use a full table scan since no index exists.
Small number of rows
The cost of full table scan is less than index range scan due to small table.
When query processed SELECT COUNT, nulls existed in the column
The query is counting the number of null columns in a typical index. However, SELECT COUNT can't count the number of null columns.
The query is unselective
The number of return rows is too large and takes nearly 100% in the whole table. These rows are unselective.
The table statistics does not update
The number of rows in the table is higher than before, but table statistics haven't been updated yet. The optimizer can't correctly estimate that using the index is faster.
The high degree of parallelism table distorts the optimizer from a true way, because optimizer would use full table scan.
A full table scan hint
The hint lets optimizer to use full table scan.
Example
A full table scan example: The example shows the SQLstatement of searching items with id is bigger than 10 from table1 SELECT category_id1 FROM table1 WHERE category_id2 > 10; In this situation, the database system needs to scan full table to find the content which fits the requirement. The other example shows the SQL statement of searching employee information by their first name order SELECT first_name FROM employees ORDER BY first_name; In this situation, the database system also needs to scan full table to compare the first name.
Pros and Cons
Pros:
The cost is predictable, as every time database system needs to scan full table row by row.
When table is less than 2 percent of database block buffer, the full scan table is quicker.
Cons:
Full table scan occurs when there is no index or index is not being used by SQL. And the result of full scan table is usually slower that index table scan. The situation is that: the larger the table, the slower of the data returns.
Unnecessary full-table scan will lead to a huge amount of unnecessary I/O with a process burden on the entire database.
