Creating temporary table while still accessing other tables - c++

Question

I've got 2 or more databases ATTACHed to one SQLite database connection. Each database consists of 3 tables. To have better access for searching/filtering, I create a huge temporary table over all tables and databases like this:
"CREATE TEMPORARY TABLE temp_table AS "
"SELECT * FROM pb.name_table "
"LEFT JOIN pb.phone_table ON (pb.name_table.id=pb.phone_table.id) " \
"LEFT JOIN pb.email_table ON (pb.name_table.id=pb.email_table.id) " \
"UNION SELECT * FROM name_table " \<br>
"LEFT JOIN phone_table ON (name_table.id=phone_table.id) " \
"LEFT JOIN email_table ON (name_table.id=email_table.id);";
If something changes in a table, I have to recreate the temporary table. With an increasing amount of data, creating a temporary table takes some time, but since I'm having continuous read access to the table, my idea was as follows:
Create a thread, which creates a second temp table in background
Block access for the reading clients
DROP first temp table
Rename the second temp table
The problem is now: Creating a temporary table is a write access to the database, which blocks automatically all reading threads also.
Has anyone a good idea how I can handle this? I need read access while recreating the temporary table.

Answer 1

As long as all threads are part of the same program, you have control over all database connections.
So you can write the data in a completely separate database, and ATTACH is quickly later.

Answer 2

Thanks a lot for your answer. I changed my code and found out that I need a new database connection (sqlite3_open) in the working thread not to block the other thread. Also creating a TEMPORARY TABLE in the attached "temporary database" was not possible, because creating a temporary table doesn't allow a qualifier (like: x.temp_table), so I had to create a real table which consumes a lot of memory in our flash file system (which is not allowed).
But wait! I've got an idea
(2 hours later)
I did it! I open an empty database and attach alll relevant databases to the connection. I create a temporary table "in" the empty database which consumes no memory in flash, because it's temporary.
When I have to create a new table, I open another empty database and attach the relevant databases to the connection. When the operation is finished, I switch the old and the new connection. Code as follows:
#include <iostream>
#include "dbAccess.h"
#include <cstdio>
#include <string>
#include <cstring>
using namespace std;
bool inProgress = true;
DWORD WINAPI createTempTableThread(void *param);
int callback(void *NotUsed, int argc, char **argv, char **azColName)
{
cout << "*";
return 0;
}
int main(void)
{
sqlite3* db = NULL;
HANDLE hThreadHandle = NULL;
CdbAccess *dba = new CdbAccess();
int i = 0;
db = dba->dbaConnect();
dba->dbaSetDatabase(db);
cout << "INFO: Creating initial temporary table. " << endl;
sqlite3_exec(dba->dbaGetDatabase(), "CREATE TEMPORARY TABLE temp_table AS " \
"SELECT * FROM pb.name_table " \
"LEFT JOIN pb.phone_table ON (pb.name_table.id=pb.phone_table.id) " \
"LEFT JOIN pb.email_table ON (pb.name_table.id=pb.email_table.id) " \
"UNION SELECT * FROM intern.name_table " \
"LEFT JOIN intern.phone_table ON (intern.name_table.id=intern.phone_table.id) " \
"LEFT JOIN intern.email_table ON (intern.name_table.id=email_intern.table.id);", NULL, NULL, NULL);
cout << "INFO: Creating initial temporary table finished. " << endl;
while(1)
{
hThreadHandle = CreateThread(0, 0, createTempTableThread, dba, 0, 0);
while(inProgress)
{
sqlite3_exec(dba->dbaGetDatabase(), "SELECT * FROM name_table WHERE id LIKE 1;", callback, NULL, NULL);
}
for(i = 0; i < 5; i++)
{
sqlite3_exec(dba->dbaGetDatabase(), "SELECT * FROM name_table WHERE id LIKE 2;", callback, NULL, NULL);
}
inProgress = true;
CloseHandle(hThreadHandle);
}
dba->dbaDisconnect();
return 0;
}
CdbAccess::CdbAccess()
{
hSemaphore = CreateSemaphore(NULL, 1, 1, 0);
}
CdbAccess::~CdbAccess()
{
}
sqlite3 *CdbAccess::dbaConnect()
{
sqlite3 *db;
static int num = 1;
int err = SQLITE_OK;
string attach = "ATTACH DATABASE \"";
string internal = "cbInternal.db";
if(num == 1)
{
cout << endl << "INFO: cbTemp1.db";
err = sqlite3_open("cbTemp1.db", &db);
num = 2;
}
else
{
cout << endl << "INFO: cbTemp2.db";
err = sqlite3_open("cbTemp2.db", &db);
num = 1;
}
if(err == SQLITE_OK)
{
cout << endl << "INFO: Temp database opened.";
err = sqlite3_exec(db, "ATTACH DATABASE \"cbInternal.db\" AS intern;", NULL, NULL, NULL);
if(err == SQLITE_OK)
{
cout << endl << "INFO: Internal database attached.";
err = sqlite3_exec(db, "ATTACH DATABASE \"0123456789.db\" AS pb;", NULL, NULL, NULL);
if(err == SQLITE_OK)
{
cout << endl << "INFO: Phone book attached.";
}
else
{
cout << endl << "ERROR: Attaching phone book: " << sqlite3_errmsg(db);
}
}
else
{
cout << endl << "ERROR: Attaching internal database: " << sqlite3_errmsg(db);
}
}
else
{
cout << endl << "ERROR: Opening database: " << sqlite3_errmsg(db);
}
return db;
}
int CdbAccess::dbaDisconnect(void)
{
int err = SQLITE_OK;
err = sqlite3_exec(db, "DETACH DATABASE pb;", NULL, NULL, NULL);
if(err == SQLITE_OK)
{
cout << endl << "INFO: Phone book detached.";
err = sqlite3_exec(db, "DETACH DATABASE intern;", NULL, NULL, NULL);
if(err == SQLITE_OK)
{
cout << endl << "INFO: Internal database detached.";
err = sqlite3_close(db);
if(err == SQLITE_OK)
{
cout << endl << "INFO: Database connection closed.";
}
else
{
cout << endl << "ERROR: Could not close database: " << sqlite3_errmsg(db);
}
}
else
{
cout << endl << "ERROR: Could not detach internal database: " << sqlite3_errmsg(db);
}
}
else
{
cout << endl << "ERROR: Could not detach phone book: " << sqlite3_errmsg(db);
}
return err;
}
sqlite3* CdbAccess::dbaGetDatabase(void)
{
return db;
}
void CdbAccess::dbaSetDatabase(sqlite3 * sqldb)
{
db = sqldb;
}
int CdbAccess::dbaGetTempTableAccess(void)
{
cout << endl << "INFO: Access requested.";
WaitForSingleObject(hSemaphore, INFINITE);
return 0;
}
int CdbAccess::dbaReleaseTempTableAccess(void)
{
cout << endl << "INFO: Access released.";
ReleaseSemaphore(hSemaphore, 1, NULL);
return 0;
}
DWORD WINAPI createTempTableThread(void *param)
{
int err = SQLITE_OK;
CdbAccess *d = (CdbAccess *)param;
sqlite3 *db;
cout << endl << "INFO: createTempTable: IN";
inProgress = true; // global variable for test porpose only
db = d->dbaConnect();
if(db != NULL)
{
cout << endl << "Thread: INFO: Creating temporary table. ";
err = sqlite3_exec(db, "CREATE TEMPORARY TABLE temp_table AS " \
"SELECT * FROM pb.name_table " \
"LEFT JOIN pb.phone_table ON (pb.name_table.id=pb.phone_table.id) " \
"LEFT JOIN pb.email_table ON (pb.name_table.id=pb.email_table.id) " \
"UNION SELECT * FROM intern.name_table " \
"LEFT JOIN intern.phone_table ON (intern.name_table.id=intern.phone_table.id) " \
"LEFT JOIN intern.email_table ON (intern.name_table.id=intern.email_table.id);", NULL, NULL, NULL);
}
if(err != SQLITE_OK)
{
cout << endl << "Thread: ERROR: Creating temporary table: " << sqlite3_errmsg(db);
}
else
{
cout << endl << "Thread: INFO: Creating temporary table finished. ";
}
d->dbaSetDatabase(db);
inProgress = false; // global variable for test porpose only
cout << endl << "Thread: INFO: createTempTable: OUT";
return 0;
}