<s>
In	O
various	O
SQL	B-Language
implementations	O
,	O
a	O
hint	O
is	O
an	O
addition	O
to	O
the	O
SQL	B-Language
standard	O
that	O
instructs	O
the	O
database	B-General_Concept
engine	I-General_Concept
on	O
how	O
to	O
execute	O
the	O
query	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
hint	O
may	O
tell	O
the	O
engine	O
to	O
use	O
or	O
not	O
to	O
use	O
an	O
index	B-Data_Structure
(	O
even	O
if	O
the	O
query	B-Language
optimizer	I-Language
would	O
decide	O
otherwise	O
)	O
.	O
</s>
<s>
Different	O
database	B-General_Concept
engines	I-General_Concept
use	O
different	O
approaches	O
in	O
implementing	O
hints	O
.	O
</s>
<s>
MySQL	B-Application
uses	O
its	O
own	O
extension	O
to	O
the	O
SQL	B-Language
standard	O
,	O
where	O
a	O
table	B-Application
name	O
may	O
be	O
followed	O
by	O
,	O
or	O
keywords	O
.	O
</s>
<s>
Oracle	B-General_Concept
implements	O
hints	O
by	O
using	O
specially-crafted	O
comments	O
in	O
the	O
query	O
that	O
begin	O
with	O
a	O
symbol	O
,	O
thus	O
not	O
affecting	O
SQL	B-Language
compatibility	O
.	O
</s>
<s>
EDB	O
Postgres	B-Application
Advanced	O
Server	O
(	O
a	O
proprietary	O
version	O
of	O
PostgreSQL	B-Application
from	O
EnterpriseDB	B-Language
)	O
offers	O
hints	O
compatible	O
with	O
those	O
of	O
Oracle	B-General_Concept
.	O
</s>
