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Went searching and didn't find the answer to this specific noob question. My apologies if I missed it.
In a MySQL database I have a table with the following primary key
PRIMARY KEY id (invoice, item)
In my application I will also frequently be selecting on "item" by itself and less frequently on only "invoice". I'm assuming I would benefit from indexes on these columns.
MySQL does not complain when I define the following:
INDEX (invoice), INDEX (item), PRIMARY KEY id (invoice, item)
But I don't see any evidence (using DESCRIBE -- the only way I know how to look) that separate indexes have been established for these two columns.
So the question is, are the columns that make up a primary key automatically indexed individually? Also, is there a better way than DESCRIBE to explore the structure of my table?
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The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
If you specify the columns in the right order in the index definition, a single composite index can speed up several kinds of queries on the same table. A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns.
The primary key are the column(s) that serves to identify the rows. An index is a physical concept and serves as a means to locate rows faster, but is not intended to define rules for the table.
Primary keys that have more than one column are always automatically indexed as composite indexes with their columns in the order that they appear in the table definition, not in the order that they are specified in the primary key definition.
I'm not intimately familiar with the internals of indices on mySql, but on the two database vendor products that I am familiar with (MsSQL, Oracle) indices are balanced-Tree structures, whose nodes are organized as a sequenced tuple of the columns the index is defined on (In the Sequence Defined)
So, unless mySql does it very differently, (probably not), any composite index (on more than one column) can be useable by any query that needs to filter or sort by a subset of the columns in the index, as long as the list of columns is compatible, i.e., if the columns, when sequenced the same as the sequenced list of columns in the complete index, is an ordered subset of the complete set of index columns, which starts at the beginning of the actual index sequence, with no gaps except at the end...
In other words, this means that if you have an index on (a,b,c,d) a query that filters on (a), (a,b), or (a,b,c) can also use the index, but a query that needs to filter on (b), or (c) or (b,c) will not be able to use the index...
So in your case, if you often need to filter or sort on column item alone, you need to add another index on that column by itself...
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