有几个方面需要考虑。

1. 能否走索引，是操作符是否支持对应的索引访问方法来决定的。

例如
检索是否走索引
排序是否走索引

pg_amop 
Name    Type    References    Description
oid    oid         Row identifier (hidden attribute; must be explicitly selected)
amopfamily    oid    pg_opfamily.oid    The operator family this entry is for
amoplefttype    oid    pg_type.oid    Left-hand input data type of operator
amoprighttype    oid    pg_type.oid    Right-hand input data type of operator
amopstrategy    int2         Operator strategy number
amoppurpose    char         Operator purpose, either s for search or o for ordering
amopopr    oid    pg_operator.oid    OID of the operator
amopmethod    oid    pg_am.oid    Index access method operator family is for
amopsortfamily    oid    pg_opfamily.oid    The B-tree operator family this entry sorts according to, if an ordering operator; zero if a search operator

2. 是否用索引是优化器决定的.

    如果走索引的成本低，可以走索引。   
    或者使用了开关，禁止全表扫，也可以走索引。