Hi, I tried to define two additional callbacks to support CustomScan on readfuncs.c.
First of all, we need to pay attention how to treat output of TextOutCustomScan when additional text output is generated. Only custom-scan provider knows what shall be displayed, so all we can do is invoke a new callback: TextReadCustomScan(). Because private fields shall be displayed next to the common field of CustomScan, this callback is invoked once pg_strtok pointer reached to the last field of CustomScan. Then, custom scan provider allocates CustomScan or a structure embedding CustomScan; only CSP knows exact size to be allocated. It can fetch some tokens using pg_strtok and reconstruct its private fields, but no need to restore the common field because it shall be done by _readCustomScan. If no callbacks are defined, _readCustomScan allocates a CustomScan object and read only known fields. Then, let's look back a bit. Next issue is how to reproduce the "methods" pointer from the text representation. I try to lookup the methods table using a pair of library and symbol name; probably, it is a straightforward way. The "methods" field is put earlier than all private fields generated by TextOutCustomScan, so it should be reconstructable prior to TextReadCustomScan. To support this feature, I had to add an interface contract that requires extensions to put library and symbol name on CustomScanMethods table. Although INIT_CUSTOM_SCAN_METHODS() macro can set up these fields, author of extension needs to pay attention. In addition to these enhancement, a new NodeCopyCustomScan callback eliminates a restriction; custom-scan provider cannot define its own structure that embeds CustomScan. Only CSP knows exact size of the structure, this callback is intended to allocate a new one and copy the private fields, but no need to copy the common fields. These three callbacks (one existing, two new) will make CustomScan node copyObject, nodeToString and stringToNode aware. I also noticed that some static functions are valuable for extensions, to avoid reinvention of same routines. How about to expose these functions? - _outToken - _outBitmapset - nullable_string - _readBitmapset The attached patch is conceptual, just compilable but not tested yet. I'll try to make a test case soon, however, it makes sense to get feedbacks earlier even if it is based on the concept design. Thanks, -- NEC Business Creation Division / PG-Strom Project KaiGai Kohei <kai...@ak.jp.nec.com>
custom-scan-on-readfuncs.v1.patch
Description: custom-scan-on-readfuncs.v1.patch
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