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Improve index-only scans to avoid repeated access to the index page.

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We copy all the matched tuples off the page during _bt_readpage, instead of
expensively re-locking the page during each subsequent tuple fetch.  This
costs a bit more local storage, but not more than 2*BLCKSZ worth, and the
reduction in LWLock traffic is certainly worth that.  What's more, this
lets us get rid of the API wart in the original patch that said an index AM
could randomly decline to supply an index tuple despite having asserted
pg_am.amcanreturn.  That will be important for future improvements in the
index-only-scan feature, since the executor will now be able to rely on
having the index data available.
This commit is contained in:
Tom Lane
2011-10-09 00:21:08 -04:00
parent 45401c1c25
commit cbfa92c23c
8 changed files with 136 additions and 135 deletions
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11
doc/src/sgml/indexam.sgml
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@ -394,12 +394,13 @@ amgettuple (IndexScanDesc scan,
If the access method supports index-only scans (i.e.,
<structfield>amcanreturn</structfield> is TRUE in its <structname>pg_am</>
row), then on success it must also check
<literal>scan-&gt;xs_want_itup</>, and if that is true it should return
<literal>scan-&gt;xs_want_itup</>, and if that is true it must return
the original indexed data for the index entry, in the form of an
<structname>IndexTuple</> stored at <literal>scan-&gt;xs_itup</>. However,
it is permissible for the access method to sometimes fail to provide this
data, in which case it must set <literal>scan-&gt;xs_itup</> to NULL. That
will result in a regular heap fetch occurring.
<structname>IndexTuple</> pointer stored at <literal>scan-&gt;xs_itup</>.
(Management of the data referenced by the pointer is the access method's
responsibility. The data must remain good at least until the next
<function>amgettuple</>, <function>amrescan</>, or <function>amendscan</>
call for the scan.)
</para>
<para>