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A full table scan is a slow but thorough way to scan a database, used when no index is present. It reads each row and checks for a query or condition set by the admin. An index column records information about the table based on admin preferences, helping to find information in the database. Full parsing may work faster for large queries affecting 5% or more of the database. The “EXPLAIN PLAN” function helps admins determine if an index is needed for a query.
A full table scan is one of the slowest, but most thorough, ways to scan a database. The reason this scanning method is so slow is that the database reads each row and checks if one or more columns are valid for a query or condition set by the administrator. The most common trigger for a full table scan is that the database does not contain an index row from which the query can be started. It’s usually best to scan the table manually if you’re searching for one row, but if you’re searching more or if you’re looking across a large table, the full table scan will work better.
Databases have scanning functions that examine tables to find information or to check the table to make sure it can satisfy a condition. A full table scan is the most thorough scan because even if it finds a row that matches the condition, it will still scan the rest of the rows. This feature is expensive, which means it uses a lot of memory, because the scan reads and writes to each row, and also does multiple searches to check the row accurately. A full scan is required when a query, or function, is executed, but the database does not see an index to execute the query.
An index is a column in a table that indexes information about the table based on administrator preferences. For example, if the administrator wants to index information about names, the index column will record the location of each name. This helps both the admin and the database to find the information hidden deep in the database. If no index is present on the table, a full table scan is usually started automatically to enable a query to be run.
So much memory is allocated to a full table scan and it’s so full that a full scan is very slow. If the administrator only needs one or a few rows for the query, it is usually faster to create a small index. While this is true, if the query is large, full parsing may actually work faster. If the administrator is running a query that affects 5% or more of the entire database, a full scan will usually be able to perform the function faster than the administrator doing the same thing manually.
An administrator will often use an “EXPLAIN PLAN” function, which will do a quick scan of the table. This will show the admin if an index is needed for the query. By taking the results, the admin will know if the query can be run as-is, if it should create or search an index, or if performing a full table scan will be efficient.
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