Improve the performance of fetching data from relation db

[Linary]

[Linary]

fetch batch_size 1000

vertices has been loaded           : 2000000
edges has been loaded           : 865500
--------------------------------------------------
total vertex metrics
parse:
    parse success           : 2000000
    parse failure           : 0
    parse time              : 10m 34s
    parse rate              : 3152(vertices/s)
load:
    load success            : 2000000
    load failure            : 0
    load time               : 10m 35s
    load rate               : 3148(vertices/s)
--------------------------------------------------
total edge metrics
parse:
    parse success           : 865500
    parse failure           : 0
    parse time              : 2m 33s
    parse rate              : 5651(edges/s)
load:
    load success            : 865500
    load failure            : 0
    load time               : 2m 33s
    load rate               : 5639(edges/s)