Some more thoughts on this topic. Let's say I have a table:
CREATE table1
primarykeyinteger INTEGER PRIMARY KEY,
timestamp1 TIMESTAMP,
payload1 VARCHAR(128),
payload2 VARCHAR(128)
CREATE INDEX myindex1 ON table1 (timestamp1)
According to tests with EXPLAIN, I found that the number of pages accessed
with:
select primarykeyinteger
from table1 where timestamp1 >= {ts '200-02-09 11:00:00' }
is equal to this query
select *
from table1 where timestamp1 >= {ts '200-02-09 11:00:00' }
Ok, I'm not trained in DB theory... but it seems to me that the I/O
pagecount should not be the same? However, EXPLAIN is giving me the same
pagecount. even though the first query is not fetching the
payload1/payload2 fields?
Questions:
1. Is Explain only telling me the pagecount for reading the index, not
the pagecount of fetching the actual fields from the table?
2. Does MaxDB not realize that it can just get the primarykeyintger
values reading directly from the index - and does't have to go actually read
the record? A feature 'not yet implemented'?
3. Is there some name for 'fetch values only from index and not records'
in database theory terminology?
Thank you.
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