update and inherits

>i want to : update virtual_table set field1 = 3 where index = 4;
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>create table virtual_table(index int4, field1 int4);
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>create table table1 ( ....)inherits(virtual_table);
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>create table table2( ....)inherits(virtual_table);
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>insert into table1(index, field1) values(4, 0);
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>and i want to change the value of field1
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>