materialize

Greetings,

I was wondering why do we need the Materialize node in the plan below when i explain a query?

1: QUERY PLAN = "Nested Loop (cost=10.99..24.34 rows=1 width=846)" (typeid = 25, len = -1, typmod = -1, byval = f)
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1: QUERY PLAN = " Join Filter: (("inner".cover)::text = ("outer".cover)::text)" (typeid = 25, len = -1, typmod = -1, byval = f)
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1: QUERY PLAN = " -> Index Scan using idx_cover_ftn_ on c_cover_ftn (cost=0.00..9.30 rows=2 width=86)" (typeid = 25, len = -1, typmod = -1, byval = f)
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1: QUERY PLAN = " Index Cond: (username = 'user1'::name)" (typeid = 25, len = -1, typmod = -1, byval = f)
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1: QUERY PLAN = " -> Materialize (cost=10.99..11.89 rows=90 width=842)" (typeid = 25, len = -1, typmod = -1, byval = f)
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1: QUERY PLAN = " -> Seq Scan on books (cost=0.00..10.90 rows=90 width=842)" (typeid = 25, len = -1, typmod = -1, byval = f)
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Thanks,
Ehab

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