Re: [SQL] making 'like' queries quicker - Mailing list pgsql-sql

A general rule of thumb is that indexes
only work on exact matches.

The reason why indexes are faster is because they store
a pointer to each of the actual records in a table in
a separate file. Each pointer is associated with a a number
which corresponds to the record's "hashed value."

E.g. if you have a record which contains a field
whose value is "dog ate my sandal," then, when the record
is indexed, the hashing algorith calculates a number
based on the letters in the value, which is the hashed
value (imagine adding the letters together to arrive
at a sum) *. That hashed value is then stored in a
hash or btree (in Postgres) index.

When you search for a word, if an index search is used,
the same hashing algorhithm is used to arrive at a hash
value for the search term.
Then all records with the same hash value are checked.
If you are looking for "dog ate my sandal," then the
hash value for your search term and the record in the
database is the same, so that record is returned.

Other values, such as "oh oh, I ate my dog," can
have the same hash value, depending on the algorithm
used. If multiple records are found with the index
search, each one is checked in turn for an exact match.

The moral of the story is that because the hashed value
only matches for records with identical data, and some
random records, while other records in the database
are not even looked at, using an index when
you don't have an exact value known won't work.

Because postgres allows function return values to be
used as a basis for an index, it is possible to find
"non-exact matches." E.g.  using a lowercase value of a
value to calculate an index. "lower(fieldname)" is
a valid "column name" to use when building an index in
Postgres. Similarly you could use a function which
only returns the first three letters of the value,
if those are the only letters you care about. You would
have to write a C function to do that, and then
use that when creating the index. You cannot use
SQL functions for creating an index in Postgres.
Built-in functions, on the other hand, (e.g. substring())
could be used, but I believe the parser can't handle them
if they have more than one argument. I could be wrong on
this. If I am, I would love to be enlightened on this
by somebody.

Just to elaborate on this a bit, it is possible to have
other schemes for calculating the hash value, with more
complexity, such as finding all words in a value, and then
adding an entry into an index for each of the matching
records. This type of approach would take more space, and
would be slower to maintain, but it would allow searches of
individuals words within strings. Postgres doesn't have
an indexing scheme like this built-in. This was just an
example of what you could do with an index.

I don't know all the internals of how indexes are
implemented in Postgres, so if I am off, please
correct me.


Troy


* A sum is not used.
  The algorithm calculates a value which has the
  best possible distribution of values within a
  certain range, so the index has an even distribution
  of values in all areas.


>
> Whatabout queries which only end with a wildcard? is there any way to
> accelerate such a query?
>
> > > Is there a way to make queries using the 'like' operator quicker, more
> > > specifically for queries that look like:
> > > select name from table where name like '%abc%';
> >
> > These kinds of queries (where the search string starts with a wildcard)
> > never use indexes, so you're stuck with the sequential scan. A faster
> > computer is probably your best option. :(
>
>
> ************
>
>