[postgis-tickets] a bug when table row count larger than 0 and less than 5

[Sandro Santilli]

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--strk;

On Fri, Mar 24, 2017 at 11:02:42AM +0800, zhuodao.zwj wrote:
> I have a table contain a column of geometry TYPE like this:
>  
> ```
> gds=# \d testgeom;
>   Column  |   Type   | Modifiers
> ----------+----------+-----------
>  the_geom | geometry |
> ```
>  
> The table testgeom has 3 records and I run the sql as follow:
>  
> ```
>  select the_geom from testgeom where ST_Intersects(the_geom,ST_GeomFromText( 'POLYGON((80.4748535 32.2128011,80.4858398 32.2128011,80.4858398 32.2035053,80.4748535 32.2035053,80.4748535 32.2128011))',4326))
>                                                                                )
> ```
>  
> Unfortunately, it raise a core:
>  
> ```
> #0  0x00007f081fca48a5 in raise () from /lib64/libc.so.6
> #1  0x00007f081fca6085 in abort () from /lib64/libc.so.6
> #2  0x00000000008f7e5e in ExceptionalCondition (
>     conditionName=0xa86098 "!(rel->rows > 0 || ((rel)->cheapest_total_path != ((void *)0) && (((((const Node*)(((rel)->cheapest_total_path)))->type) == T_AppendPath) && ((AppendPath *) ((rel)->cheapest_total_path))->subpaths == "..., errorType=0xa8600b "FailedAssertion", fileName=0xa86000 "allpaths.c", lineNumber=350)
>     at assert.c:54
> #3  0x00000000006d5930 in set_rel_size (root=0x2c6c138, rel=0x2c834c0, rti=5, rte=0x2c7bc60) at allpaths.c:350
> ```
> I find rel->rows is not a number . I gdb it and find that the zero is the divisor in the gserialized_estimate.c:1899:
> 
> ```
> cell_size[d] = (max[d] - min[d]) / nd_stats->size[d]; 
> ```
> 
> The nd_stats->size comes from the column stanumbersN table pg_statistic as follow:
>  
> ```
> stanumbers1 | {2,0,0,0,0,80.4356,32.1458,0,0,80.5024,32.3317,0,0,7,7,7,7,1,7,7}
> ```
> 
> So I find function compute_gserialized_stats_mode  in the gserialized_estimate.c:1425
>  
> ```
> histo_cells_target = Min(histo_cells_target, (int)(total_rows/5));
> ```
> 
> When total_rows is greater than 0 and less than 5, histo_cells_target will be zero and it will cause the error.
>  
> But I wonder why this histo_cells_target can be zero. Is it a bug ? Can I fix it by this:
>  
> ```
> histo_cells_target = Min(histo_cells_target, Max(1, (int)(total_rows/5)));
> ```

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