Similar simple query slow down dramatically, by just select one more field, why?

[Oscar Yen]

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‰¦ºx†j×­˜úèDear all,

I have asked the question days before, but no one seems interested in it....

Considering table imgstore, defined as

create table a (
  imgid int not null,
  parent int,
  imgtype char(3),
  img longtext,
  primary key (imgid),
  key searchkey (parent, imgid)
) type = innodb;

contains about 10000+ rows, about 800M in size;

SQL tested:

T1) select imgid, parent from a where parent = 10;
   returns 3357 rows in 0.08 sec.
T2) select imgid, parent, imgtype from a where parent = 10;
   return 3357 rows in 8.32 sec.!!!

T3) select imgid, parent from a where parent = 10 order by imgid;
   returns 3357 rows in 0.1 sec
T4) select imgid, parent, imgtype from a where parent = 10 order by imgid;
   returns 3357 rows in 25.88 sec!!!

T5) create table za select imgid, parent, imgtype from a;
   10102 rows in 1.71 sec.
T6) select imgid, parent, imgtype from za where parent = 10 order by imgid;
   3357 rows in 0.02 sec.

T7) After change the searchkey to (parent, imgid, imgtype), T1/T2, T3/T4 runs in 
almost same speed.

Anybody can explain my questions:

Q1- What cause the slow down, T2 vs T1 and T4 vs T3?
Q2- Can I assume mysql pooly handled on large BLOB data, by comparing performance of 
T6 and T4?

Best Reguards.