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| author | Tom Lane <tgl@sss.pgh.pa.us> | 2021-07-06 18:10:11 -0400 |
|---|---|---|
| committer | Tom Lane <tgl@sss.pgh.pa.us> | 2021-07-06 18:10:20 -0400 |
| commit | 955b3e0f9269639fb916cee3dea37aee50b82df0 (patch) | |
| tree | 395cf4010f26d0e3f2383f9ad724cb683135acce /doc/src | |
| parent | 5798ca529935698ab976780565fb2b4d8d34d810 (diff) | |
| download | postgresql-955b3e0f9269639fb916cee3dea37aee50b82df0.tar.gz postgresql-955b3e0f9269639fb916cee3dea37aee50b82df0.zip | |
Allow CustomScan providers to say whether they support projections.
Previously, all CustomScan providers had to support projections,
but there may be cases where this is inconvenient. Add a flag
bit to say if it's supported.
Important item for the release notes: this is non-backwards-compatible
since the default is now to assume that CustomScan providers can't
project, instead of assuming that they can. It's fail-soft, but could
result in visible performance penalties due to adding unnecessary
Result nodes.
Sven Klemm, reviewed by Aleksander Alekseev; some cosmetic fiddling
by me.
Discussion: https://postgr.es/m/CAMCrgp1kyakOz6c8aKhNDJXjhQ1dEjEnp+6KNT3KxPrjNtsrDg@mail.gmail.com
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/custom-scan.sgml | 13 |
1 files changed, 10 insertions, 3 deletions
diff --git a/doc/src/sgml/custom-scan.sgml b/doc/src/sgml/custom-scan.sgml index 239ba29de72..93d96f2f566 100644 --- a/doc/src/sgml/custom-scan.sgml +++ b/doc/src/sgml/custom-scan.sgml @@ -71,10 +71,17 @@ typedef struct CustomPath <para> <structfield>path</structfield> must be initialized as for any other path, including the row-count estimate, start and total cost, and sort ordering provided - by this path. <structfield>flags</structfield> is a bit mask, which should include + by this path. <structfield>flags</structfield> is a bit mask, which + specifies whether the scan provider can support certain optional + capabilities. <structfield>flags</structfield> should include <literal>CUSTOMPATH_SUPPORT_BACKWARD_SCAN</literal> if the custom path can support - a backward scan and <literal>CUSTOMPATH_SUPPORT_MARK_RESTORE</literal> if it - can support mark and restore. Both capabilities are optional. + a backward scan, <literal>CUSTOMPATH_SUPPORT_MARK_RESTORE</literal> if it + can support mark and restore, + and <literal>CUSTOMPATH_SUPPORT_PROJECTION</literal> if it can perform + projections. (If <literal>CUSTOMPATH_SUPPORT_PROJECTION</literal> is not + set, the scan node will only be asked to produce Vars of the scanned + relation; while if that flag is set, the scan node must be able to + evaluate scalar expressions over these Vars.) An optional <structfield>custom_paths</structfield> is a list of <structname>Path</structname> nodes used by this custom-path node; these will be transformed into <structname>Plan</structname> nodes by planner. |