Time costs of indexes

The second cost of an index is time whenever the table is modified.

The following descriptions assume that approximately two pages must be read to locate an index entry. That is the case when the index consists of a root page, one level of branch pages, and a set of leaf pages. The root page is assumed to be in a buffer already. The index for a very large table has at least two intermediate levels, so about three pages are read when the database server references such an index.

Presumably, one index is used to locate a row being altered. The pages for that index might be found in page buffers in shared memory for the database server. However, the pages for any other indexes that need altering must be read from disk.

Under these assumptions, index maintenance adds time to different kinds of modifications, as the following list shows:
  • When you delete a row from a table, the database server must delete its entries from all indexes.

    The database server must look up the entry for the deleted row (two or three pages in) and rewrite the leaf page. The write operation to update the index is performed in memory, and the leaf page is flushed when the least recently used (LRU) buffer that contains the modified page is cleaned. This operation requires two or three page accesses to read the index pages if needed and one deferred page access to write the modified page.

  • When you insert a row, the database server must insert its entries in all indexes.

    The database server must find a place in which to enter the inserted row within each index (two or three pages in) and rewrite (one deferred page out), for a total of three or four immediate page accesses per index.

  • When you update a row, the database server must look up its entries in each index that applies to an altered column (two or three pages in).

    The database server must rewrite the leaf page to eliminate the old entry (one deferred page out) and then locate the new column value in the same index (two or three more pages in) and the row entered (one more deferred page out).

Insertions and deletions change the number of entries on a leaf page. Although virtually every pagents operation requires some additional work to deal with a leaf page that has either filled or been emptied, if pagents is greater than 100, this additional work occurs less than 1 percent of the time. You can often disregard it when you estimate the I/O impact.

In short, when a row is inserted or deleted at random, allow three to four added page I/O operations per index. When a row is updated, allow six to eight page I/O operations for each index that applies to an altered column. If a transaction is rolled back, all this work must be undone. For this reason, rolling back a transaction can take a long time.

Because the alteration of the row itself requires only two page I/O operations, index maintenance is clearly the most time-consuming part of data modification. For information about one way to reduce this cost, see Clustering.


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