From: drh <>
Date: Tue, 9 Mar 2021 19:52:15 +0000 (+0000)
Subject: Improved comments on the NOT NULL strength reduction optimization.
X-Git-Tag: version-3.35.0~20
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=45e4cb8e775af60599ab81b1f5c261bb2d176447;p=thirdparty%2Fsqlite.git

Improved comments on the NOT NULL strength reduction optimization.

FossilOrigin-Name: a85d72293914b48edbb39171fd591d37ffb09570d8103140a052203ec71d49ee
---

diff --git a/manifest b/manifest
index acab927fec..2ef9cf8d39 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Extra\scomments\sand\stestcase()\smacros\sassociated\swith\sthe\snew\nsqlite3ParserAddCleanup()\smechanism.\s\sNo\schanges\sto\srelease\sbuilds.
-D 2021-03-09T19:32:37.488
+C Improved\scomments\son\sthe\sNOT\sNULL\sstrength\sreduction\soptimization.
+D 2021-03-09T19:52:15.121
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -540,7 +540,7 @@ F src/pragma.h 8dc78ab7e9ec6ce3ded8332810a2066f1ef6267e2e03cd7356ee00276125c6cf
 F src/prepare.c e21b54489b5c73b06ada15e6fc79b5c6f64b06701924a6ca98944ae59e06256f
 F src/printf.c 2b03a80d7c11bb422115dca175a18bf430e9c9dbaa0eee63b758f0c022f8f34f
 F src/random.c 80f5d666f23feb3e6665a6ce04c7197212a88384
-F src/resolve.c 688070848f0a0c41bcc545a4b4b052921d9abc29ba3102985d3d6f7595d9637c
+F src/resolve.c d95db73d3e6a5c689e5f6604b4d2521350e45f2a0f0f84f5a2dc2bfee56580a0
 F src/rowset.c ba9515a922af32abe1f7d39406b9d35730ed65efab9443dc5702693b60854c92
 F src/select.c fc904a7aa7ebfd5c7a57a0141d829c9f5388ac7773e0d1d9668768c1bbc87fc3
 F src/shell.c.in af18a2e980aabe739a8188266464866fe7947b100674e07480e7ba3e37595947
@@ -1910,7 +1910,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 85d28b52250e1817b722dc10da3b7b73abf7539cbe7aa913b5960ae4da40d5f8
-R d17cbf155a86829dd5e4cb02d43d6be1
+P c3c8691121e6cb7bfc326212cc81a7472c0e6f22e655ab024bd4bd885e1cd878
+R 5194ad2c2b4aa2abc9c790b24065e147
 U drh
-Z 544f2bbbf57a7528504952e032cce28b
+Z a0573751157ef7d8069d3366c3f1599b
diff --git a/manifest.uuid b/manifest.uuid
index b7e814f171..76500de6bd 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-c3c8691121e6cb7bfc326212cc81a7472c0e6f22e655ab024bd4bd885e1cd878
\ No newline at end of file
+a85d72293914b48edbb39171fd591d37ffb09570d8103140a052203ec71d49ee
\ No newline at end of file
diff --git a/src/resolve.c b/src/resolve.c
index 668498cb96..32914befb2 100644
--- a/src/resolve.c
+++ b/src/resolve.c
@@ -788,16 +788,19 @@ static int resolveExprStep(Walker *pWalker, Expr *pExpr){
       break;
     }
 
-    /* An "<expr> IS NOT NULL" or "<expr> IS NULL". After resolving the
-    ** LHS, check if there is a NOT NULL constraint in the schema that
-    ** means the value of the expression can be determined immediately.
-    ** If that is the case, replace the current expression node with 
-    ** a TK_TRUEFALSE node.
+    /* An optimization:  Attempt to convert
     **
-    ** If the node is replaced with a TK_TRUEFALSE node, then also restore
-    ** the NameContext ref-counts to the state they where in before the
-    ** LHS expression was resolved. This prevents the current select
-    ** from being erroneously marked as correlated in some cases.
+    **      "expr IS NOT NULL"  -->  "TRUE"
+    **      "expr IS NULL"      -->  "FALSE"
+    **
+    ** if we can prove that "expr" is never NULL.  Call this the
+    ** "NOT NULL strength reduction optimization".
+    **
+    ** If this optimization occurs, also restore the NameContext ref-counts
+    ** to the state they where in before the "column" LHS expression was
+    ** resolved.  This prevents "column" from being counted as having been
+    ** referenced, which might prevent a SELECT from being erroneously
+    ** marked as correlated.
     */
     case TK_NOTNULL:
     case TK_ISNULL: {