[tarantool-patches] Re: [PATCH v2 1/5] sql: introduce structs assembling DDL arguments during parsing

[Vladislav Shpilevoy]

On 29/01/2019 23:04, n.pettik wrote:
> 
> 
>> On 29 Jan 2019, at 22:29, Vladislav Shpilevoy <v.shpilevoy at tarantool.org> wrote:
>>
>> Hi! Thanks for the patchset! See 5 comments below.
>>
>>> What is more, we are going to introduce ALTER TABLE ADD CONSTRAINT
>>> UNIQUE With new hierarchy we can extend ALTER TABLE statement with ease:
>>> basic structures (alter -> create entity -> create constraint) are the
>>> same for .. FOREIGN KEY/UNIQUE, but the last one will be different.
>>> Note that grammar for CREATE TABLE statement is not trivial and consists
>>> of wide range of sub-clauses (e.g. to parse foreign key or check
>>> constraints). Therefore, parts of space definition are assembled
>>> as soon as parser processes sub-rules. For this reason, current patch
>>> doesn't affect CREATE TABLE handling.
>>
>> 1. Why? I do not think, that "create table" port into parse_def.h is
>> impossible and follows from the facts above. 'Not trivial' does
>> not mean impossible.
>>
>> As I see, you can create struct create_table_def, which will contain
>> Table *pNewTable, struct rlist new_fkey, uint32_t fkey_count,
>> bool is_new_table_autoinc. Just move some attributes from struct Parse
>> to this new structure. Is it possible? I think, it is worth doing,
>> because the code will look more consistent - any object change will go
>> through parse_def.h structures.
> 
> Ok, it is possible.
> 
>>> +struct alter_entity_def {
>>> +	/** As a rule it is a name of table to be altered. */
>>> +	struct SrcList *entity_name;
>>> +};
>>> +
>>> +struct rename_entity_def {
>>> +	struct alter_entity_def *base;
>>
>> 2. Please, look at how we usually do inheritance in C.
>> We include base structure into a child as an attribute,
>> not as a pointer. So you can cast the child to the parent
>> even without touching 'base' field. Also, this change
>> together with comment 4 allows to remove parse_def.c
>> completely.
> 
> Surely, before implementing this patch I checked out
> inheritance examples in our code base. But here we
> have slightly different case (or I misunderstood concepts).
> 
> For instance, when we create memtx space, we allocate
> enough space for base space and for memtx parent at
> once. When we are parsing ALTER TABLE, we are able
> to allocate memory only for base structure (alter_def),
> since we don’t know where we will get - whether to
> drop constraint or create constraint or rename.
> If we used similar inheritance model, we would
> have to realloc enough memory (for base + parent)
> each time we get to the next rule.

You haven't, if struct Parse stores a union of all
terminal defs instead of a pointer. Then this field
will have enough memory to fit any def, with all its
base attributes. See the comment 4 from the original
email. And the comment below.

> 
> What I mean:
> 
> alter_table_start ::= …
>    struct alter_entity_def *alter_def = malloc(sizeof(alter_def));
> ….
> 
> Then we can get to “rename” rule or to “add_constraint_start”.
> 
> rename ::=
> …
> struct rename_entity_def *rename_def = malloc (sizeof(alter_def));
> memcpy(rename_def, alter_def, sizeof(alter_def));
> free(alter_def);
> …
> 
> The same allocation and free procedures would be called
> inside add_constraint_start.
> 
> Or, as an option, we can intensely use region allocation.
> But then we are going to waste a lot of memory.
> 
> Is it what you talked about? Or I am wrong and you meant
> easier way?
> 

No, you have no realloc anything, if you just remove '*' from
all 'base' members and use union of all terminal symbols
in struct Parse as I described. Then this union member in struct
Parse will have enough memory to fit any terminal symbol, with
all its inheritance levels (because of 'base' attributes, stored
by value).

It slightly similar to struct port. Base struct port has
padding, which is able to fit any child attributes, so we
can create base struct port, and then 'develop' it to a more
concrete port without reallocs. Here you will have a similar
situation, if fix my comment 4.