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engine condition pushdown
by playing with a SQL-statement I encountered a problem. I'm not sure
if this is a bug or if I did not use the correct method to get the
information I expected.
So let me please explain the situation in which I encountered this
problem. But it will take some lines of text to come to the point.
My environment: MariaDB 10.0.10 source-code-version on Linux.
The statement: select SQL_NO_CACHE B.PZN, B.ArtikelText from
TestSmall B where exists ( select 1 from TestBig A where A.Hersteller =
'36367' and A.PZN = B.PZN) and B.Hersteller = '00020'
Engine used is MyISAM but the effect should be independent of any
engine (not checked). No indexes exist on the tables involved.
You will find a lot more of details here: http://augustq.blogspot.com/2
Execution of this (silly) statement is done by a table-scan on the
table TestSmall. When a record is found the matches the WHERE-condition
the server switches over to the table TestBig and does a table-scan on
this table. When a match is found it returns to the table TestSmall,
otherwise the table TestBig is read to the end and then the server
returns to the table TestSmall. The server continues scanning the table
TestSmall, switching over to TestBig, returning to TestSmall, continues
reading TestSmall and so on, until it reaches the end of the table
Of interest for me was the condition tree which can be inspected by
looking at the function cond_push() (in my case
So let's start.
The function cond_push() is initally called for the tables TestSmall
and TestBig, this looks good.
Then the server starts reading the table TestSmall and therefore the
function cond_push() is called again. This looks good too.
Now a matching record is found in TestSmall: it switches over to the
table TestBig and calls cond_push() with the condition-tree for this
table. This tree looks like this:
COND-ITEM args: 0 type=[COND_AND_FUNC]
FUNC-ITEM [=] args: 2 type=[EQ_FUNC]
FIELD-ITEM [TestOpt] [A] [Hersteller] name=<Hersteller>
STRING-ITEM str_length=<5> str_value=<36367> name=<36367>
FUNC-ITEM [=] args: 2 type=[EQ_FUNC]
FIELD-ITEM [TestOpt] [A] [PZN] name=<PZN>
FIELD-ITEM [TestOpt] [B] [PZN] name=<PZN>
The last line is of interest (I've marked it in bold). For the table
with the alias B (= TestSmall) no value is given for the column PZN but
this value is available. This is the first point of interest.
Let's continue with the operation. When the table TestBig is read to
the end, the scanning of TestSmall continues and a matching record is
found over there the server again switches over to TestBig. Before it
starts reading the records from this table the function cond_push() is
called again. This is the output of the condition-tree, restricted to
the line of interest (the last line):
FIELD-ITEM [TestOpt] [B] [PZN]
str_length=<2> str_value=<37> name=<PZN>
So let's continue: when it switches to TestBig for the 3rd time this
output looks like:
FIELD-ITEM [TestOpt] [B] [PZN] str_length=<7> str_value=<222 >
Enough of data. I can deliver more examples if needed or you may look
into my text.
The output of the first case des not show the value of the column PZN
from the current record in TestSmall although the value is available.
The output in the second case shows the value 37 (because of the length
eq. 2) but the correct value would be 3717968 (and length eq. 7).
The output in the third case shows the value 222 followed by 4 spaces
(length eq. 7), but the correct value is 222 (length eq. 3).
What I see is the correct value (exclude the first example presented
here) but the length-information is one row behind (one row of
For accessing the information presented here I used COND->str_value.
So the question is:
- the information is available but I used the wrong
- is this a bug in the code?
What do you think?
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