what is difference between lazy="true" and fetch="select" in hibernate?

To solve n+1 select problem for n queries (parent child relationship) in hibernate we use fetch="join" instead of fetch="select" . Lazy setting decides whether to load child objects while loading the parent object. You need to do this setting respective hibernate mapping file of the parent class. lazy="true" means not to load the child. By default the lazy loading of the child objects is true.

Yes.

The lazy attribute tells hibernate when to get the children.

The fetch attribute tells hibernate how to get the children.

When you say

The lazy=true attribute is enable lazy loading of the parent and child collections and same thing fetch="select" attribute

that is flat out incorrect. The select fetch strategy is NOT the same thing as turning lazy loading off. In fact, from the documentation

Select fetching: a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you access the association.

When we say fetch="select", then it will always fire separate queries to retrieve the association objects even if it is lazy ="false".

But when we say lazy ="true", it means that it will retrieve the association objects in a separate query, but not at the time of loading the object, but when the association is first accessed. We can do it by saying list().size().

When we say fetch="join" it will always fire a single query to get the association objects from the database.