Bruce Momjian wrote:
Bruce Momjian wrote:
Bruce Momjian wrote:
Also, I am wondering whether the information that which index is used to
fetch a tuple is always available. I haven't checked, but do we have that
information in lossy bitmap heapscan ?
Oh, that is an interesting problem because an index might have one index
entry representing an entire HOT chain, while another index might
represent each chain member by individual index entries. When we do the
bitmaps, don't we access them by heap tid, meaning we would find all
entries anyway?
I thinking some more, it would be a problem because while we are merging
the tids, we are using index entries and haven't looked at the heap yet.
I am guessing we would have to exclude the new index from bitmap joins
with other indexes until the VACUUM happens.
Thinking some more, bitmap scans have a mode that tracks just the page
numbers, rather than the tids --- if the index visibilities do not
match, we would need to fall back to that mode.
You don't need to scan the whole page like in the lossy bitmap mode,
just all the tuples in the HOT-chain.
You need to somehow pass the information that multiple indexes have been
used in the bitmap scan to the bitmap heapscan node, so that it knows
when the extra checking is required.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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