SQL Anywhere uses physical and logical indexes. A physical index is the actual indexing structure as it is stored on disk. A logical index is a reference to a physical index. When you create a primary key, secondary key, foreign key, or unique constraint, the database server ensures referential integrity by creating a logical index for the constraint. Then, the database server looks to see if a physical index already exists that satisfies the constraint. If a qualifying physical index already exists, the database server points the logical index to it. If one does not exist, the database server creates a new physical index and then points the logical index to it. For a physical index to satisfy the requirements of a logical index, the columns, column order, and the ordering (ascending, descending) of data for each column must be identical. Information about all logical and physical indexes in the database is recorded in the ISYSIDX and ISYSPHYSIDX system tables, respectively. When you create a logical index, an entry is made in the ISYSIDX system table to hold the index definition. A reference to the physical index used to satisfy the logical index is recorded in the ISYSIDX.phys_id column. The physical index is defined in the ISYSPHYSIDX system table. Using logical indexes means that the database server does not need to create and maintain duplicate physical indexes since more than one logical index can point to a single physical index. When you delete a logical index, its definition is removed from the ISYSIDX system table. If it was the only logical index referencing a particular physical index, the physical index is also deleted, along with its corresponding entry in the ISYSPHYSIDX system table. Physical indexes are not created for remote tables. For temporary tables, physical indexes are created, but they are not recorded in ISYSPHYSIDX, and are discarded after use. Also, physical indexes for temporary tables are not shared. Discuter à propos de cette page dans DocCommentXchange. |Copyright © 2013, SAP AG ou société affiliée SAP - SAP Sybase SQL Anywhere 16.0|